Liquid crystal display/optical retardation compensator combination in which variations in the dispersion of light in the liquid crystal and/or in the compensator materials minimize undesired screen coloration

ABSTRACT

The liquid crystal display device is composed of at least one optical retardation compensator plate  2  (and  3 ) inserted between a liquid crystal display element 1 and polarizer plates  4  and  5 . The liquid crystal display element  1  is composed of a pair of electrode substrates  6  and  7  and a liquid crystal layer  8  sealed therebetween. The polarizer plates  4  and  5  flank the liquid crystal display element  1 . The optical retardation compensator plate  2  (and  3 ) has a negative refractive index anisotropy (na=nc&gt;nb) . The direction of a principal refractive index nb parallel to the normal to the surface and the direction of either a principal refractive index na or nc in the surface incline either clockwise or counterclockwise around the direction of the principal refractive index nc or na in the surface. Moreover, either the pretilt angle formed by the orientation films  11  and  14  and the longer axes of liquid crystal molecules in the liquid crystal layer or the value of applied voltage for displaying halftone obtained by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal is set within such a range that tone reversion does not occur in the opposite viewing direction when halftone is being displayed.

FIELD OF THE INVENTION

[0001] The present invention relates to a liquid crystal display device,especially, to a liquid crystal display device with the viewing angledependency of the display screen abated by a combination of a liquidcrystal display element and an optical retardation compensator plate.

BACKGROUND OF THE INVENTION

[0002] Conventionally, liquid crystal display devices incorporatingnematic liquid crystal display elements have been in widespread use fornumeral-segment-type display devices such as watches and calculators,and recently the applications are finding more places with wordprocessors, notebook-type personal computers, liquid crystal televisionsmounted in automobiles, etc.

[0003] Generally, a liquid crystal display element has a translucentsubstrate, electrode lines for turning on and off pixels, and othercomponents formed on the substrate. For example, in an active-matrixtype liquid crystal display device, active elements, such as thin-filmtransistors, are formed on the substrate together with the electrodelines as switching means for selectively driving pixel electrodes bywhich voltages are applied across the liquid crystal. Moreover, inliquid crystal display devices capable of color display, color filterlayers having colors such as red, green and blue are provided on thesubstrate.

[0004] Liquid crystal display elements such as the one mentioned aboveadopt a liquid crystal display mode that is suitably selected dependingon the twist angle of the liquid crystal: some of well-known modes areactive-driving-type twisted nematic liquid crystal display mode(hereinafter, referred to as the TN mode) and the multiplex-driving-typesuper-twisted nematic liquid crystal display mode (hereinafter, referredto as the STN mode).

[0005] The TN mode displays images by orientating the nematic liquidcrystal molecules to a 90°-twisted state so as to direct rays along thetwisted directions. The STN mode utilizes the fact that thetransmittance is allowed to change abruptly in the vicinity of thethreshold value of the applied voltage across the liquid crystal byexpanding the twist angle of the nematic liquid crystal molecules to notless than 90°.

[0006] The problem with the STN mode is that the background of thedisplay screen sustains a peculiar color due to interference betweencolors because of the use of the birefringence effect of liquid crystal.In order to solve this problem and to provide a proper black-and-whitedisplay in the STN mode, the application of an optical retardationcompensator plate is considered to be effective. Display modes using theoptical retardation compensator plate are mainly classified into twomodes, that is, the double layered super-twisted nematicoptical-retardation compensation mode (hereinafter, referred to as theDSTN mode) and the film-type optical-retardation compensation mode(hereinafter, referred to as the film-addition mode) wherein a filmhaving optical anisotropy is provided.

[0007] The DSTN mode uses a two-layered construction that has adisplay-use liquid crystal cell and a liquid crystal cell which areorientated with a twist angle in a direction opposite to that of thedisplay-use liquid crystal cell. The film-addition mode uses aconstruction wherein a film having optical anisotropy is disposed. Here,the film-addition mode is considered to be more prospective in thestandpoint of light weight and low costs. Since the application of suchan optical-retardation compensation mode makes it possible to improveblack-and-white display characteristics, color STN liquid crystaldisplay devices have been achieved that enable color display byinstalling color-filter layers in STN-mode display devices.

[0008] The TN modes are, on the other hand, classified into the NormallyBlack mode and the Normally White mode. In the Normally Black mode, apair of polarizer plates are placed with their polarization directionsin parallel with each other, and black display is provided in a statewhere no ON voltage is applied across the liquid crystal layer (OFFstate). In the Normally White mode, a pair of polarizer plates areplaced with their polarization directions orthogonal to each other, andwhite display is provided in the OFF state. Here, the Normally Whitemode is considered to be more prospective from the standpoints ofdisplay contrast, color reproducibility, viewing angle dependency, etc.

[0009] However, in the TN-mode liquid crystal display device, liquidcrystal molecules have a refractive index anisotropy An, and areorientated so as to incline to the above and below substrates. For thesereasons, the viewing angle dependency increases: i.e., the contrast ofdisplayed images varies depending upon the direction and angle of theviewer.

[0010]FIG. 11 schematically shows the cross-sectional construction of aTN liquid crystal display element 31. This state shows liquid crystalmolecules 32 slanting upward slightly as a result of application of avoltage for halftone display. In such a liquid crystal display element31, a linearly polarized ray 35 passing through the surfaces of a pairof substrates 33 and 34 along the normals thereto, and linearlypolarized rays 36 and 37 passing through those surfaces not along thenormals thereto cross the liquid crystal molecules 32 at differentangles. Besides, the liquid crystal molecules 32 have a refractive indexanisotropy An. Therefore, the linearly polarized rays 35, 36 and 37,upon passing through the liquid crystal molecules 32 in differentdirections, produce ordinary and extraordinary rays. The linearlypolarized rays 35, 36 and 37 are converted to elliptically polarizedrays according to the phase difference between the ordinary andextraordinary rays, which cause the viewing angle dependency.

[0011] In addition, in an actual liquid crystal layer, the liquidcrystal molecules 32 show different tilt angles in the vicinity of themidpoint between the substrates 33 and 34 and in the vicinities of thesubstrates 33 and 34. The liquid crystal molecules 32 are twisted by 90°around the normal.

[0012] For those reasons described so far, the linearly polarized rays35, 36 and 37 passing through the liquid crystal layer are affected bythe birefringence effect in various ways depending upon, for example,the directions and the angles thereof, resulting in complex viewingangle dependency.

[0013] Such viewing angle dependency can be observed, as examples, inthe following situations. If the viewing angle increases from the normalto the display screen in the standard viewing direction, i.e. downward,and exceeds a certain angle, the displayed image has a distinct color(hereinafter, referred to as the coloration phenomenon), or is reversedin black and white (hereinafter, referred to as the tone reversionphenomenon). If the viewing angle increases from the normal in theopposite viewing direction, i.e. upward, the contrast decreasesabruptly.

[0014] The aforementioned liquid crystal display device has anotherproblem that the effectual range of viewing angle narrows with a largerdisplay screen. When a large liquid crystal display device is viewedfrom a short distance in the front thereof, the same color may appeardifferent in the uppermost and lowermost parts of the large screen dueto the effect of the viewing angle dependency. This is caused by a widerrange of viewing angle required to encompass the whole screen surface,which is equivalent to a viewing direction which is increasingly far offcenter.

[0015] To restrain the viewing angle dependency, Japanese Laid-OpenPatent Applications No. 55-600/1980 (Tokukaisho 55-600) and No.56-97318/1981 (Tokukaisho 56-97318) suggest that an optical retardationcompensator plate (retardation compensator film) be inserted as anoptical element having optical anisotropy between the liquid crystaldisplay element and one of polarizer plates.

[0016] According to the method, the elliptically polarized ray convertedfrom a linearly polarized ray by passing through liquid crystalmolecules having refractive index anisotropy is directed through theoptical retardation compensator plate(s) disposed on the side(s) of theliquid crystal layer having refractive index anisotropy. Hence, thephase difference between the ordinary and extraordinary rays whichoccurs to the viewing angle are compensated for, and the ellipticallypolarized ray is converted back to the linearly polarized ray, whichenables the restraint of the viewing angle dependency.

[0017] Japanese Laid-Open Patent Application No. 5-313159/1993(Tokukaihei 5-313159), as an example, discloses an optical retardationcompensator plate of the above kind represented by a refractive indexellipsoid with one of the principal refractive indices parallel to thenormal to the surface of the optical retardation compensator plate.Nevertheless, this optical retardation compensator plate still cannotsatisfactorily restrain the tone reversion phenomenon that occurs whenthe viewing angle increases in the standard viewing direction.

[0018] In order to eliminate the tone reversion phenomenon, JapaneseLaid-Open Patent Application No. 57-186735/1982 (Tokukaisho 57-186835)discloses the so-called pixel dividing method, in which a displayedpattern (pixel) is divided and orientation is controlled so that eachdivided segment has its own viewing angle characteristics independentfrom those of the other segments. According to the method, since theliquid crystal molecules stand upwards in different directions fromsegment to segment, the viewing angle dependency can be eliminated.However, the problem of a lower contrast when the viewing angleincreases upward or downward cannot be solved.

[0019] Japanese Laid-Open Patent Applications No. 6-118406/1994(Tokukaihei 6-118406) and No. 6-194645/1994 (Tokukaihei 6-194645)disclose technologies to combine the pixel dividing method and anoptical retardation compensator plate.

[0020] The liquid crystal display device disclosed in Japanese Laid-OpenPatent Application No. 6-118406/1994 includes an optical anisotropicfilm (optical retardation compensator plate) interposed between theliquid crystal panel and the polarizer plate to, for example, improvethe contrast. The retardation compensator plate (optical retardationcompensator plate) disclosed in Japanese Laid-Open Patent ApplicationNo. 6-194645/1994 is set to have almost no phase difference in a planeparallel to the surface of the retardation compensator plate and to havea larger refractive index in a plane perpendicular to the surface of theretardation compensator plate than the refractive index in a planeparallel thereto, in order to have a negative refractive index.Therefore, when a voltage is applied, the positive refractive indexoccurring to the liquid crystal display element is compensated for andviewing angle dependency can be decreased.

[0021] Nevertheless, the application of the pixel dividing method to theuse of this optical retardation compensator plate still fails touniformly restrain the decrease in contrast in the vertical directions;coloration phenomenon still occurs in oblique directions when theviewing angle is 45°.

[0022] For these reasons, there are limits to the restraining of thecontrast variation, coloration phenomenon, and tone reversion phenomenonrelated with viewing angle, by means of a retardation compensator platerepresented by a refractive index ellipsoid positioned upright, i.e., arefractive index ellipsoid with one of the principal refractive indicesthereof parallel to the normal to the surface of the retardationcompensator plate.

[0023] Hence, Japanese Laid-Open Patent Application No. 6-75116/1994(Tokukaihei 6-75116) suggests the use of an optical retardationcompensator plate represented by a refractive index ellipsoid with theprincipal refractive indices inclining to the normal to the surface ofthe optical retardation compensator plate. This method adopts two kindsof optical retardation compensator plates as follows.

[0024] One of the optical retardation compensator plates can berepresented by such a refractive index ellipsoid that the smallest ofthe three principal refractive indices is parallel to the surface, oneof the two larger principal refractive indices inclines to the surfaceof the optical retardation compensator plate by an angle θ, theremaining principal refractive index inclines to the normal to theoptical retardation compensator plate by the same angle θ, and the angleθ satisfies 20°≦θ≦70°.

[0025] The other optical retardation compensator plate can berepresented by a refractive index ellipsoid inclining to the surface,where the three principal refractive indices, na, nb, and nc, aremutually related by the inequality na=nc>nb, and the direction of theprincipal refractive index nb parallel to the normal to the surface andthe direction of either the principal refractive index na or nc in thesurface recline either clockwise or counterclockwise around thedirection of the principal refractive index nc or na in the surface.

[0026] As for the former optical retardation compensator plate, auniaxial and biaxial optical retardation compensator plate can be used.For the latter one, two optical retardation compensator plates, insteadof one, can be used in such a combination that the two principalrefractive indices nb form an angle of 90°.

[0027] A liquid crystal display device, incorporating at least one suchoptical retardation compensator plate between the liquid crystal displayelement and the polarizer plate exhibits some restraint in the contrastvariations, coloration phenomenon, and tone reversion phenomenon causedby the viewing angle dependency of the display screen.

[0028] However, with today's increasingly large demand on a widereffectual range of viewing angle and superb display quality, a betterrestraint in the viewing angle dependency is crucial. In this context,the optical retardation compensator plate disclosed in JapaneseLaid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) abovedoes not provide satisfactory solutions and needs to be improved.

SUMMARY OF THE INVENTION

[0029] In view of the above problems, the first object of the presentinvention is, on top of the improvement by the compensation effects bythe optical retardation compensator plate, to restrain the viewing angledependency, and especially, to effectively restrain the tone reversionin the opposite viewing direction when halftone is being displayed byapplying a voltage that is close to the threshold voltage for the liquidcrystal.

[0030] The second object of the present invention is, on top of theimprovement by the compensation effects by the optical retardationcompensator plate, to restrain the viewing angle dependency, andespecially, to effectively restrain the coloration phenomenon.

[0031] In order to accomplish the first object, a liquid crystal displaydevice of the first arrangement in accordance with the present inventionincludes:

[0032] a liquid crystal display element formed by sealing a liquidcrystal layer between a pair of substrates;

[0033] a pair of polarizers disposed so as to flank the liquid crystaldisplay element; and

[0034] at least one optical retardation compensator plate disposedbetween the liquid crystal display element and the polarizers, theoptical retardation compensator plate being represented by an incliningrefractive index ellipsoid,

[0035] wherein the pretilt angle formed by the orientation films and thelonger axes of liquid crystal molecules in the liquid crystal layer isset within such a range that tone reversion does not occur in theopposite viewing direction when halftone is being displayed by applyingto the liquid crystal a voltage that is close to the threshold voltagefor the liquid crystal.

[0036] As explained above, the first arrangement of the presentinvention incorporates, between the liquid crystal layer and thepolarizer, an optical retardation compensator plate represented by aninclining refractive index ellipsoid. Therefore, with the arrangement,for a case where a linearly polarized ray is converted to anelliptically polarized ray according to the phase difference between theordinary and extraordinary rays developed from the linearly polarizedray upon the passing through the liquid crystal layer possessingbirefringence, the optical retardation compensator plate compensates forthe phase difference between the ordinary and extraordinary rays thatvaries depending upon the viewing angle.

[0037] With the liquid crystal display device of the first arrangementin accordance with the present invention, the pretilt angle of theliquid crystal layer sealed in the liquid crystal display element is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal. This can eliminate the tone reversion in the oppositeviewing direction on a screen displaying halftone, and thereby furtherrestrain the viewing angle dependency of the screen. The contrastvariations and coloration are also restrained better than only by thecompensation function by the optical retardation compensator plate.

[0038] In the first arrangement above, the abrupt decrease in luminancecan be restrained in the standard viewing direction when halftone isbeing displayed, by further setting the pretilt angle within such arange that luminance does not decrease abruptly in the standard viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal.

[0039] For these reasons, with the arrangement, the contrast ratio inblack and white display is not affected by the viewing angle of theobserver, and the quality of images displayed by the liquid crystaldisplay device is greatly improved.

[0040] In order to accomplish the first object, a liquid crystal displaydevice of the second arrangement in accordance with the presentinvention includes:

[0041] a liquid crystal display element formed by sealing a liquidcrystal layer between a pair of substrates;

[0042] a pair of polarizers disposed so as to flank the liquid crystaldisplay element; and

[0043] at least one optical retardation compensator plate disposedbetween the liquid crystal display element and the polarizers, theoptical retardation compensator plate being represented by an incliningrefractive index ellipsoid,

[0044] wherein the value of applied voltage for displaying halftoneobtained by applying to the liquid crystal a voltage that is close tothe threshold voltage for the liquid crystal is set within such a rangethat tone reversion does not occur in the opposite viewing directionwhen halftone is being displayed.

[0045] Even if a linearly polarized ray is converted to an ellipticallypolarized ray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, thesecond arrangement compensates for the phase difference by the opticalretardation compensator plate similarly to the first arrangement.

[0046] With the liquid crystal display device of the second arrangement,the value of applied voltage for displaying halftone obtained byapplying to the liquid crystal a voltage that is close to the thresholdvoltage for the liquid crystal is set within such a range that tonereversion does not occur in the opposite viewing direction when halftoneis being displayed. This can eliminate the tone reversion in theopposite viewing direction with a screen displaying halftone, andthereby further restrain the viewing angle dependency of the screen. Thecontrast variations and coloration are also restrained better than onlyby the compensation function by the optical retardation compensatorplate.

[0047] In the second arrangement above, the abrupt decrease in luminancecan be restrained in the standard viewing direction when halftone isbeing displayed, by further setting the value of applied voltage fordisplaying halftone obtained by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal withinsuch a range that luminance does not decrease abruptly in the standardviewing direction when halftone is being displayed.

[0048] For these reasons, with the arrangement, the contrast ratio inblack and white display is not affected by the viewing angle of theobserver, and the quality of images displayed by the liquid crystaldisplay device is greatly improved.

[0049] In order to accomplish the second object, a liquid crystaldisplay device of the third arrangement in accordance with the presentinvention includes:

[0050] a liquid crystal display element formed by sealing a liquidcrystal layer between a pair of substrates;

[0051] a pair of polarizers disposed so as to flank the liquid crystaldisplay element; and

[0052] at least one optical retardation compensator plate disposedbetween the liquid crystal display element and the polarizers, theoptical retardation compensator plate being represented by an incliningrefractive index ellipsoid,

[0053] wherein the ratios of the variation in the refractive indexanisotropy, Δn_(L), of the liquid crystal material for the liquidcrystal layer with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light are set within such arange that viewing angle dependency does not cause coloration on theliquid crystal screen.

[0054] Even if a linearly polarized ray is converted to an ellipticallypolarized ray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, thearrangement compensates for the phase difference by the opticalretardation compensator plate, similarly to the first arrangement.

[0055] With the liquid crystal display device of the third embodiment,the ratios of the variation in the refractive index anisotropy, Δn_(L),of the liquid crystal material for the liquid crystal layer with thewavelength of light and of the variation in the refractive indexanisotropy, Δn_(F), of the optical retardation compensator plate withthe wavelength of light are set within such a range that viewing angledependency does not cause coloration on the liquid crystal screen. Thiscan further restrain coloration on the screen. The contrast variationsand coloration are also restrained better than only by the compensationfunction by the optical retardation compensator plate.

[0056] Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device is preferably arrangedso that the refractive index anisotropy, Δn_(L)(550), of the liquidcrystal material for the liquid crystal layer to light having awavelength of 550 nm is set within a range larger than 0.060 and smallerthan 0.120.

[0057] The setting can eliminate the phase difference that occurs to theliquid crystal display element in accordance with the viewing angle.That can further restrain the contrast variations and tone reversionphenomenon in the right- and left-hand directions, as well as thecoloration phenomenon that occurs depending upon the viewing angle.

[0058] In such an event the phase difference that occurs to the liquidcrystal display element in accordance with the viewing angle can be moreeffectively eliminated by setting the refractive index anisotropy,Δn_(L)(550), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 550 nm so as to be within a range notsmaller than 0.070 and not larger than 0.095. This can surely restrainthe contrast variations and tone reversion phenomenon in the right- andleft-hand directions of the images displayed by the liquid crystaldisplay device.

[0059] Moreover, in the first, second, and third arrangements, theliquid crystal display device is preferably arranged so that the or eachoptical retardation compensator plate is represented by a refractiveindex ellipsoid inclining by an inclination angle θ set within a rangeof 15° to 75°.

[0060] By setting the inclination angle of the refractive indexellipsoid to be within a range of 15° to 75° with respect to the or eachoptical retardation compensator plate incorporated in the liquid crystaldisplay device, it is assured that the present invention provides theaforementioned compensation function for the phase difference by theoptical retardation compensator plate.

[0061] Moreover, in the first, second, and third arrangements, theliquid crystal display device is preferably arranged so that the or eachoptical retardation compensator plate has a product, (n_(a)−n_(o))×d, ofthe difference between the principal refractive indices, na and nb, andthe thickness, d, of the optical retardation compensator plate, theproduct being set to be from 80 nm to 250 nm.

[0062] By setting the product, (n_(a)−n_(b))×d, of the differencebetween the principal refractive indices, na and nb, and the thickness,d, of the optical retardation compensator plate, so as to be from 80 nmto 250 nm with respect to the or each optical retardation compensatorplate incorporated in the liquid crystal display device, it is assuredthat the present invention provides the aforementioned compensationfunction for the phase difference by the optical retardation compensatorplate.

[0063] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, are not in any way intendedto limit the scope of the claims of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064]FIG. 1 is a cross-sectional view showing the arrangement of aliquid crystal display device in accordance with the first embodiment ina decomposed manner.

[0065]FIG. 2 is an explanatory drawing showing the relation between therubbing direction of the orientation film and the standard viewingdirection in the liquid crystal display device.

[0066]FIG. 3 is a perspective view illustrating the principle refractiveindices of an optical retardation compensator plate of the liquidcrystal display device.

[0067]FIG. 4 is a perspective view showing the optical arrangement of apolarizer plate and the optical retardation compensator plate of theliquid crystal display device in a decomposed manner.

[0068]FIG. 5 is an explanatory drawing showing the pretilt angle formedby the linger axes of the liquid crystal molecules and the orientationfilm.

[0069]FIG. 6 is a perspective view showing a system for measuring theviewing angle dependency of the liquid crystal display device.

[0070] FIGS. 7(a), 7(b), and 7(c) are graphs showing the transmittanceversus liquid crystal applied voltage characteristics of the liquidcrystal display devices of the first example and a comparative examplefor the first example.

[0071] FIGS. 8(a), 8(b), and 8(c) are graphs showing the transmittanceversus liquid crystal applied voltage characteristics of the liquidcrystal display devices of the second example.

[0072] FIGS. 9(a), 9(b), and 9(c) are graphs showing the transmittanceversus liquid crystal applied voltage characteristics of the liquidcrystal display devices of a comparative example for the second example.

[0073]FIG. 10 is a graph showing Δn(λ)/Δn_(L)(550), for wavelengths λ oflight, of an optical retardation compensator plate and a liquid crystalmaterial used as the liquid crystal layer of the liquid crystal displaydevice.

[0074]FIG. 11 is a schematic view showing the twisted orientation ofliquid crystal molecules in an TN liquid crystal display element.

DESCRIPTION OF THE EMBODIMENTS

[0075] First Embodiment

[0076] The following description will discuss the first embodiment inaccordance with the present invention.

[0077] As illustrated in FIG. 1, the liquid crystal display device ofthe present embodiment is provided with a liquid crystal display element1, a pair of optical retardation compensator plates 2 and 3, and a pairof polarizer plates (polarizers) 4 and 5.

[0078] The liquid crystal display element 1 is constituted by electrodesubstrates 6 and 7 that are placed face to face with each other and aliquid crystal layer 8 that is sandwiched therebetween. The electrodesubstrate 6 is constructed as follows: a glass substrate (a translucentsubstrate) 9 is provided as a base, a transparent electrode 10, made ofITO (Indium Tin Oxide), is formed on the surface, of the glass substrate9, facing the liquid crystal layer 8, and an orientation film 11 isformed on the transparent electrode 10. The electrode substrate 7 isconstructed; a glass substrate (a translucent substrate) 12 is providedas a base, a transparent electrode 13, made of ITO, is formed on thesurface, of the glass substrate 12, facing the liquid crystal layer 8,and an orientation film 14 is formed on the transparent electrode 13.

[0079] Although FIG. 1 shows a construction corresponding to two pixelsfor convenience of explanation, the transparent electrodes 10 and 13,which are strips with a predetermined width, are respectively placed onthe glass substrates 9 and 12 with predetermined intervals all over theliquid crystal display element 1, and are designed so that they areorthogonal to each other on the glass substrates 9 and 12, when viewedin a direction perpendicular to the substrate surfaces. Portions atwhich the transparent electrodes 10 and 13 intersect each othercorrespond to pixels for carrying out display, and the pixels are placedin a matrix format over the entire structure of the present liquidcrystal display device.

[0080] The electrode substrates 6 and 7 are bonded by seal resin 15, anda liquid crystal layer 8 is sealed inside the space formed by the sealresin 15 and the electrode substrates 6 and 7. A voltage is applied viathe transparent electrodes 10 and 13 by a driving circuit (voltageapplication means) 17 according to display data.

[0081] The pretilt angle of the liquid crystal layer 8 of the presentliquid crystal display device is set so as to produce the bestproperties when combined with the compensation function for phasedifference by the optical retardation compensator plates 2 and 3 (willbe described later in detail).

[0082] In the present liquid crystal display device, a unit, formed byincorporating optical retardation compensator plates 2 and 3 andpolarizer plates 4 and 5 into the above-mentioned liquid crystal displayelement 1, is referred to as a liquid crystal cell 16.

[0083] The orientation films 11 and 14 are treated with a rubbingtechnique in advance so that the liquid crystal molecules between themare orientated with a twist angle of about 90°. As shown in FIG. 2, therubbing direction R₁ of the orientation film 11 and the rubbingdirection R₂ of the orientation film 14 are set to be orthogonal to eachother.

[0084] The optical retardation compensator plates 2 and 3 are providedbetween the liquid crystal display element 1 and the respectivepolarizer plates 4 and 5 disposed to flank the liquid crystal displayelement 1. The optical retardation compensator plates 2 and 3 areconstituted by a support base made of a transparent organic polymer anddiscotic liquid crystal. The discotic liquid crystal is treated with anoblique orientation technique or hybrid orientation, and crosslinked. Asa result, the optical retardation compensator plates 2 and 3 are formedso as to have a refractive index ellipsoid (will be described later indetail) that inclines to the optical retardation compensator plates 2and 3.

[0085] As for the support base of the optical retardation compensatorplates 2 and 3, triacetylcellulose (TAC), which is generally used forpolarizer plates, is suitably applied with high reliability. Besidesthis, colorless, transparent organic polymeric films made ofpolycarbonate (PC), polyethyleneterephthalate (PET), etc., which aresuperior in environment resistance and chemical resistance, are alsosuitably applied.

[0086] As illustrated in FIG. 3, each of the optical retardationcompensator plates 2 and 3 has principal refractive indices na, nb andnc pointing in three different directions. The direction of theprincipal refractive index na coincides with the direction of they-coordinate axis among the mutually orthogonal x-, y-, and z-coordinateaxes. The direction of the principal refractive index nb inclines by θin the direction of arrow A with respect to the z-coordinate axis(parallel to a normal to the surface) that is perpendicular to thesurface of the optical retardation compensator plates 2 and 3, whichsurface corresponds to the screen.

[0087] The principal refractive indices na, nb, and nc of the opticalretardation compensator plates 2 and 3 are related to each other by theinequality: na=nc>nb. Therefore, there exists only one optic axis, andthe optical retardation compensator plates 2 and 3 have uniaxiality anda negative refractive index anisotropy. The first retardation value,(nc−na)×d, of the optical retardation compensator plates 2 and 3 equalsalmost 0 nm, since na=nc, while the second retardation value, (nc−nb)×d,is set to an arbitral value in a range from 80 nm to 250 nm. By settingthe second retardation value in such a range, the compensation functionfor phase difference by the optical retardation compensator plates 2 and3 is surely achieved. Note that (nc−na) and (nc−nb) each represent arefractive index anisotropy Δn_(F), and that d represents the thicknessof the optical retardation compensator plates 2 and 3.

[0088] The angle θ by which the direction of the principal refractiveindices nb of the optical retardation compensator plates 2 and 3incline, i.e. the inclination angle θ of the refractive indexellipsoids, is set to an arbitrary value in the range 15°≦θ≦75°. Bysetting the inclination angle θ to such a value, regardless of whetherthe refractive index ellipsoids incline clockwise or counterclockwise,the compensation function for phase difference by the opticalretardation compensator plates 2 and 3 is surely achieved.

[0089] Instead of using the two optical retardation compensator plates 2and 3, only one of them may be used and disposed on one side.Alternatively, both the optical retardation compensator plates 2 and 3can be disposed on one side, one of them overlapping the other. As afurther alternative, three or more optical retardation compensatorplates may be used.

[0090] As illustrated in FIG. 4, in the present liquid crystal displaydevice, the polarizer plates 4 and 5 in the liquid crystal displayelement 1 are arranged so that their absorption axes AX₁ and AX₂ areparallel to the rubbing directions R₁ and R₂ of the orientation films 11and 14 respectively (see FIG. 1). In the present liquid crystal displaydevice, since the rubbing directions R₁ and R₂ are orthogonal to eachother, the absorption axes AX₁ and AX₂ are also orthogonal to eachother.

[0091] Here, as illustrated in FIG. 3, the direction D is defined as adirection formed by projecting the direction of the principal refractiveindex nb, which inclines in such a direction to impart anisotropy to theoptical retardation compensator plates 2 and 3, onto the surface of theoptical retardation compensator plates 2 and 3. As illustrated in FIG.4, the optical retardation compensator plate 2 is placed so that thedirection D (direction D₁) is parallel to the rubbing direction R₁, andthe optical retardation compensator plate 3 is placed so that thedirection D (direction D₂) is parallel to the rubbing direction R₂.

[0092] With the above-mentioned arrangement of the optical retardationcompensator plates 2 and 3 and the polarizer plates 4 and 5, the presentliquid crystal display device can carry out so-called Normally Whitedisplay wherein rays of light are allowed to pass during OFF time sothat white display is provided.

[0093] In general, in optical anisotropic materials such as liquidcrystal and optical retardation compensator plates (phase differencefilms), the above-mentioned anisotropy of the three-dimensionalprincipal refractive indices na, nc and nb is represented by arefractive index ellipsoid. The refractive-index anisotropy Δn assumesdifferent values depending on which direction the refractive indexellipsoid is observed.

[0094] Next, the aforementioned setting of the pretilt angle for theliquid crystal layer 8 will be explained in detail.

[0095] As illustrated in FIG. 5, the pretilt angle is the angle θ formedby the orientation film 14 (11) and the longer axes of liquid crystalmolecules 20, and determined by the combination of rubbing treatment ofthe liquid crystal material and the rubbing of the orientation films 11and 14.

[0096] As mentioned earlier, the pretilt angle of the liquid crystallayer 8 of the present liquid crystal display device is set to producethe best properties when combined with the compensation function forphase difference by the optical retardation compensator plates 2 and 3.Specifically, the pretilt angle is set in a range that does not causetone reversion in the opposite viewing direction in a halftone displaystate where a voltage that is close to the threshold voltage for theliquid crystal is applied to the liquid crystal. Here, since theNormally White display mode is selected, the halftone display state isclose to white color. Hereinafter, the halftone display state close towhite color will be referred to as white tone.

[0097] It has been confirmed through experiments that the larger thepretilt angles are, the less likely the tone reversion occurs in theopposite viewing direction, whereas too large pretilt angles cause anabrupt decrease in luminance in the standard viewing direction whenwhite tone is being displayed. Thus, the pretilt angle also needs to beset within such a range that luminance does not decrease abruptly in thestandard viewing direction when white tone is being displayed.

[0098] More specifically, used as the orientation films 11 and 14 andthe liquid crystal material is a combination of orientation films and aliquid crystal material that results in a pretilt angle more than 2° andless than 12°. More preferable is a combination that results in apretilt angle not less than 4° and not more than 10°.

[0099] The setting of the pretilt angle in a range more than 2° and lessthan 12° enables the liquid crystal display device to be free fromproblem-posing tone reversion in the opposite viewing direction whenwhite tone is being displayed and to be viewed in every direction at theviewing angle of 50° which is typically required for liquid crystaldisplay devices.

[0100] Especially, the setting of the pretilt angle in a range not lessthan 4° and not more than 10° enables the liquid crystal display deviceto be viewed without tone reversion at all in the opposite viewingdirection at the viewing angle of 70° when white tone is beingdisplayed.

[0101] Selected as the liquid crystal material for the liquid crystallayer 8 of the liquid crystal display device in accordance with thepresent invention is a liquid crystal material of which the refractiveindex anisotropy, Δn_(L)(550), to light having a wavelength of 550 nm isdesigned to be within a range larger than 0.060 and smaller than 0.120.More preferably, a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550), is designed to be within a range not smallerthan 0.070 and not larger than 0.095 is used.

[0102] As a result, the optical retardation compensator plates 2 and 3become capable of compensating for the phase difference. And so does thesetting of the pretilt angle in the range above. Moreover, the decreasein contrast ratio in the opposite viewing direction can be furtherrestrained, and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained.

[0103] As explained so far, the liquid crystal display device of thepresent embodiment includes, between the liquid crystal display element1 and the polarizer plates 4 and 5, the optical retardation compensatorplates 2 and 3 each represented by a refractive index ellipsoid havingthree principal refractive indices, na, nb, and nc, mutually related bythe inequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

[0104] wherein the pretilt angle of the liquid crystal layer 8 is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal.

[0105] As a result, the tone reversion phenomenon that occurs in theopposite direction according to the viewing angle when white tone(because Normally White display is being adopted) is being displayed canbe, above all, effectively restrained by the compensation function forphase difference that occurs to the liquid crystal display element 1according to the viewing angle by the setting of the pretilt angle inthe range above, as well as by the compensation function by the opticalretardation compensator plates 2 and 3. Besides, the contrast variationscan be improved, resulting in display of high quality images.

[0106] Besides, the liquid crystal display device of the presentembodiment employs as the liquid crystal material for the liquid crystallayer 8 a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550), to light having a wavelength of 550 nm, isdesigned to be within a range larger than 0.060 and smaller than 0.120.Therefore, the optical retardation compensator plates 2 and 3 becomecapable of compensating for the phase difference. And so does thesetting of the pretilt angle in the range above. Moreover, the decreasein contrast ratio in the opposite viewing direction can be furtherrestrained, and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained.

[0107] Note that although the liquid crystal display device of NormallyWhite display has been taken as an example in the description above, thesame effects can be obtained with a liquid crystal display device ofNormally Black display by achieving compensation function for phasedifference by the setting of the pretilt angle within such a range thattone reversion does not occur in the opposite viewing direction whenhalftone (black tone) is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal, as well as by the compensation function by the opticalretardation compensator plates 2 and 3.

[0108] Note also that although the liquid crystal display device of asimple matrix method has been taken as an example in the description ofthe embodiment above, the present invention can be applied to a liquidcrystal display device of an active matrix method using active switchingelements such as TFTs.

[0109] Second Embodiment

[0110] The following description will discuss another embodiment inaccordance with the present invention. Here, for convenience, members ofthe present embodiment that have the same function as members of thefirst embodiment, and that are mentioned in the first embodiment areindicated by the same reference numerals and description thereof isomitted.

[0111] The liquid crystal display device of the present embodiment isconfigured almost in the same manner as is the liquid crystal displaydevice of the first embodiment shown in FIG. 1, except the followingpoints:

[0112] The liquid crystal display device of the first embodimentincludes the liquid crystal layer 8 of which the pretilt angle is set ina range that does not cause tone reversion in the opposite viewingdirection in a halftone display state where a voltage that is close tothe threshold voltage for the liquid crystal is applied to the liquidcrystal layer 8, so as to produce the best properties when combined withthe compensation function for phase difference by the opticalretardation compensator plates 2 and 3.

[0113] The liquid crystal display device of the present embodiment, bycontrast, includes a liquid crystal layer 8 such that the value of theapplied voltage for displaying halftone obtained by applying to theliquid crystal layer 8 a voltage that is close to the threshold voltagefor the liquid crystal is set within such a range that tone reversiondoes not occur in the opposite viewing direction when halftone is beingdisplayed, so as to produce the best properties when combined with thecompensation function for phase difference by the optical retardationcompensator plates 2 and 3.

[0114] Next, the above differences will be explained in detail.

[0115] Since the liquid crystal display device of the present embodimentis of Normally White display, the value of the applied voltage forrealizing halftone display state where a voltage that is close to thethreshold voltage for the liquid crystal is applied to the liquidcrystal, i.e. white tone, is set within such a range that tone reversiondoes not occur in the opposite viewing direction when that voltage isbeing applied.

[0116] It has been confirmed through experiments that the lower thetransmittance when white tone is being displayed is, the less likely thetone reversion occurs in the opposite viewing direction when white toneis being displayed. On the other hand, too low transmittances cause anabrupt decrease in luminance in the standard viewing direction and inthe right- and left-hand directions. Thus, the voltage applied to theliquid crystal that determines the transmittance when white tone isbeing displayed needs to be set also within such a range that luminancedoes not decrease abruptly in the standard viewing direction and in theright- and left-hand directions when white tone is being displayed.

[0117] Specifically, the voltage applied to the liquid crystal whenwhite tone is being displayed is set so that the transmittance whenwhite tone is being displayed is higher than 85% that in the OFF state.In such a case, the voltage applied to the liquid crystal when whitetone is being displayed is more preferably set so that the transmittancewhen white tone is being displayed is in a range not less than 90% andnot more than 97% that in the OFF state. The OFF state refers to a statewhere the voltage applied to the liquid crystal is zero.

[0118] The setting of the voltage applied to the liquid crystal whenwhite tone is being displayed so that the transmittance when white toneis being displayed is higher than 85% that in the OFF state enables theliquid crystal display device to be free from problem-posing tonereversion in the opposite viewing direction when white tone is beingdisplayed and to be viewed in every direction at the viewing angle of50° which is typically required for liquid crystal display devices.

[0119] Especially, the setting of the voltage applied to the liquidcrystal when white tone is being displayed so that the transmittancewhen white tone is being displayed is in a range not less than 90% andnot more than 97% that in the OFF state enables the liquid crystaldisplay device to be viewed without tone reversion at all in theopposite viewing direction at the viewing angle of 70° when white toneis being displayed.

[0120] As explained above, the liquid crystal display device of thepresent embodiment includes, between the liquid crystal display element1 and the polarizer plates 4 and 5, the optical retardation compensatorplates 2 and 3 each represented by a refractive index ellipsoid havingthree principal refractive indices, na, nb, and nc, mutually related bythe inequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

[0121] wherein the value of the applied voltage for realizing halftonedisplay where a voltage that is close to the threshold voltage for theliquid crystal is applied to the liquid crystal is set within such arange that tone reversion does not occur in the opposite viewingdirection in the state where that voltage is applied.

[0122] As a result, the tone reversion phenomenon that occurs in theopposite direction according to the viewing angle when white tone(because Normally White display is being adopted) is being displayed canbe, above all, effectively restrained by the compensation function forphase difference that occurs to the liquid crystal display element 1according to the viewing angle by the setting of the voltage applied tothe liquid crystal when white tone is being displayed in the rangeabove, as well as by the compensation function by the opticalretardation compensator plates 2 and 3. Besides, the contrast variationscan be improved, resulting in display of high quality images.

[0123] Besides, similarly to the liquid crystal display device of theprevious embodiment, by employing as the liquid crystal material for theliquid crystal layer 8 a liquid crystal material of which the refractiveindex anisotropy, Δn_(L)(550), to light having a wavelength of 550 nm isdesigned to be within a range larger than 0.060 and smaller than 0.120,and more preferably, within a range not smaller than 0.070 and notlarger than 0.095, the decrease in contrast ratio in the oppositeviewing direction and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained by the compensationfunction for phase difference by the setting of the voltage applied tothe liquid crystal when white tone is being displayed in the rangeabove, as well as by the compensation function by the opticalretardation compensator plates 2 and 3.

[0124] Note that although the liquid crystal display device of NormallyWhite display has been taken as an example in the description above, thesame effects can be obtained with a liquid crystal display device ofNormally Black display by achieving compensation function for phasedifference by the setting of the voltage to be applied to the liquidcrystal for halftone (black tone) display obtained by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal within such a range that tone reversion does not occur inthe opposite viewing direction when halftone is being displayed, as wellas by the compensation function by the optical retardation compensatorplates 2 and 3.

[0125] Note also that similarly to the first embodiment, apart from theliquid crystal display device of a simple matrix method, the presentinvention can be applied to a liquid crystal display device of an activematrix method using active switching elements such as TFTs.

[0126] Third Embodiment

[0127] The following description will discuss another embodiment inaccordance with the present invention. Here, for convenience, members ofthe present embodiment that have the same function as members of theprevious embodiments, and that are mentioned in the previous embodimentsare indicated by the same reference numerals and description thereof isomitted.

[0128] The liquid crystal display device of the present embodiment isconfigured almost in the same manner as is the liquid crystal displaydevice of the first embodiment shown in FIG. 1, except the followingpoints:

[0129] The liquid crystal display device of the first embodimentincludes the liquid crystal layer 8 of which the pretilt angle is set ina range that does not cause tone reversion in the opposite viewingdirection in a halftone display state where a voltage that is close tothe threshold voltage for the liquid crystal is applied to the liquidcrystal layer 8, so as to produce the best properties when combined withthe compensation function for phase difference by the opticalretardation compensator plates 2 and 3.

[0130] The liquid crystal display device of the present embodiment, bycontrast, includes a liquid crystal layer 8 such that the ratios of thevariation in the refractive index anisotropy, Δn_(L), of the liquidcrystal material for the liquid crystal layer 8 with the wavelength oflight and of the variation in the refractive index anisotropy, Δn_(F),of the optical retardation compensator plate with the wavelength oflight are set within such a range that viewing angle dependency does notcause coloration on the liquid crystal screen, so as to produce the bestproperties when combined with the compensation function for phasedifference by the optical retardation compensator plates 2 and 3.

[0131] Next, the above differences will be explained in detail.

[0132] Setting the ratios of the variation in the refractive indexanisotropy, Δn_(L), of the liquid crystal material for the liquidcrystal layer 8 with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light within such a range thatviewing angle dependency does not cause coloration on the liquid crystalscreen refers to, in more specific terms, a combined use of opticalretardation compensator plates 2 and 3 with a liquid crystal materialthat satisfy at least one of the range-setting conditions {circumflexover (1)} and {circumflex over (2)} below:

[0133] {circumflex over (1)} The ratio, Δn_(L)(450)/Δn_(L)(550), of therefractive index anisotropy, Δn_(L)(450), of the liquid crystal materialfor the liquid crystal layer 8 to light having a wavelength of 450 nmand the refractive index anisotropy, Δn_(L)(550), thereof to lighthaving a wavelength of 550 nm, and the ratio, Δn_(F)(450)/Δn_(F)(550),of the refractive index anisotropy, Δn_(F)(450), of the opticalretardation compensator plates 2 and 3 to light having a wavelength of450 nm and the refractive index anisotropy, Δn_(F)(550), thereof tolight having a wavelength of 550 nm are set to satisfy the inequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} < 0.35$

[0134] and are, more preferably, set to satisfy the inequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} \leq 0.25$

[0135] {circumflex over (2)} The ratio, Δn_(L)(650)/Δn_(L)(550), of therefractive index anisotropy, Δn_(L)(650), of the liquid crystal materialfor the liquid crystal layer 8 to light having a wavelength of 650 nmand the refractive index anisotropy, Δn_(L)(550), thereof to lighthaving a wavelength of 550 nm, and the ratio, Δn_(F)(650)/Δn_(F)(550),of the refractive index anisotropy, Δn_(F)(650), of the opticalretardation compensator plates 2 and 3 to light having a wavelength of650 nm and the refractive index anisotropy, Δn_(F)(550), thereof tolight having a wavelength of 550 nm are set to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} < 0.27$

[0136] and are, more preferably, set to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} \leq 0.20$

[0137] The use of a liquid crystal material and optical retardationcompensator plates designed to satisfy at least one of the conditions{circumflex over (1)} and {circumflex over (2)} permits the effectiverestraint in, above all, the coloration phenomenon on the displayscreen, in addition to the restraint in the contrast variations, tonereversion phenomenon, and coloration phenomenon caused by the viewingangle dependency of the display screen by the compensation function forphase difference by the optical retardation compensator plates 2 and 3.

[0138] To be more specific, although in some instances still incapableof completely eliminating coloration at the viewing angle of 50°, whichis the viewing angle typically required for liquid crystal displaydevices, satisfying at least one of the wider conditions of {circumflexover (1)} and {circumflex over (2)} enables the liquid crystal displaydevice to be viewed in every direction without problems for real use.

[0139] And, satisfying at least one of the preferred conditions of{circumflex over (1)} and {circumflex over (2)} enables the liquidcrystal display device to be viewed in every direction without anycoloration at all at the viewing angle of 70°.

[0140] Furthermore, satisfying at least one of {circumflex over (1)} and{circumflex over (2)} restrains also contrast variations and tonereversion phenomenon better than does the compensation function by theoptical retardation compensator plates 2 and 3 alone.

[0141]FIG. 10 shows Δn(λ)/Δn(550) for wavelengths λ with a combinationof a liquid crystal material that can be used as the liquid crystallayer 8 of the present liquid crystal display device and of opticalretardation compensator plates that can be used as the opticalretardation compensator plates 2 and 3. The solid curved line a showsΔn_(L)(λ)/Δn_(L)(550) for wavelengths λ of a liquid crystal material,while the alternative long and short dash line b showsΔn_(F)(λ)/Δn_(F)(550) for wavelengths λ of an optical retardationcompensator plate.

[0142] As explained above, the liquid crystal display device of thepresent embodiment includes, between the liquid crystal display element1 and the polarizer plates 4 and 5, the optical retardation compensatorplates 2 and 3 each represented by a refractive index ellipsoid havingthree principal refractive indices, na, nb, and nc, mutually related bythe inequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

[0143] wherein the ratios of the variation in the refractive indexanisotropy, Δn_(L), of the liquid crystal material for the liquidcrystal layer 8 with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light are set within such arange that viewing angle dependency does not cause coloration on theliquid crystal screen.

[0144] As a result, the coloration phenomenon caused by the viewingangle dependency of the display screen can be, above all, effectivelyrestrained by the compensation function for phase difference that occursto the liquid crystal display element 1 according to the viewing angleby the setting of the ratios of the variation in the refractive indexanisotropy, Δn_(L), of the liquid crystal material for the liquidcrystal layer 8 with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light in the range above, aswell as by the compensation function by the optical retardationcompensator plates 2 and 3. Besides, the contrast variations and tonereversion phenomenon can be improved, resulting in display of highquality images.

[0145] Besides, similarly to the liquid crystal display device of theprevious embodiments, by employing as the liquid crystal material forthe liquid crystal layer 8 a liquid crystal material of which therefractive index anisotropy, Δn_(L)(550) to light having a wavelength of550 nm, is designed to be within a range larger than 0.060 and smallerthan 0.120, and more preferably, within a range not smaller than 0.070and not larger than 0.095, the decrease in contrast ratio in theopposite viewing direction and the tone reversion phenomenon in theright- and left-hand directions can be further restrained by thecompensation function for phase difference by the setting of the ratiosof the variations in the range above, as well as by the compensationfunction by the optical retardation compensator plates 2 and 3.

[0146] Note that although the liquid crystal display device of NormallyWhite display has been taken as an example in the description above, thesame effects can be obtained with a liquid crystal display device ofNormally Black display.

[0147] Note also that similarly to the first embodiment, apart from theliquid crystal display device of a simple matrix method, the presentinvention can be applied to a liquid crystal display device of an activematrix method using active switching elements such as TFTs.

[0148] The following description will explain examples that substantiatethe effects of the liquid crystal display devices of the first, second,and third embodiments.

FIRST EXAMPLE

[0149] The present example is to substantiate the effects of the liquidcrystal display devices of the first and second embodiments. Here, sevensample cells #1 to #7 were prepared by using Optomer AL (product name),available from Japan Synthetic Rubber Co., Ltd., as the orientationfilms 11 and 14 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1, selecting suitable liquid crystalmaterials to set the pretilt angles to 2.0°, 3.0+, 4.0°, 5.0°, 10.0°,11.0°, and 12.0° with respect to the orientation films 11 and 14, andsetting the thickness of the cells of the liquid crystal layers 8 to 5μm.

[0150] Homogeneous cells were prepared by injecting thereinto thematerials for the sample cells #1 to #7, and measured with a pretiltangle measuring device, NSMAP-300OLCD (Sigma Optical Machinery Co.,Ltd.), for the pretilt angles of the sample cells #1 to #7.

[0151] Used as the optical retardation compensator plates 2 and 3 of thesample cells #1 to #7 are those constituted by a transparent supportbase (e.g., triacetylcellulose (TAC)) on which discotic liquid crystalis applied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

[0152] Tables 1 to 7 show results of visual observations of the samplecells #1 to #7 under white light with various voltages applied for whitetone. TABLE 1 Applied voltage for white tone set to derive atransmittance 100% that in the OFF state Viewing Pretilt angle (°) Angle2.0 3.0 4.0 5.0 10.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘x₂ 60° x₁ ∘ ∘ ∘ ∘ ∘ x₂ 70° x₁ Δ₂ Δ₁ ∘ ∘ x₂ x₂ # pose any problem forreal use, “x₁” represents that tone reversion was observed in theopposite viewing direction, and “x₂” represents that a decrease inluminance was evident in the standard viewing direction to the extentunbearable for real use.)

[0153] Table 1 shows, supposing that the transmittance along the normalto the surface of the liquid crystal cell 16 as 100% in an OFF statewhere the voltage applied to the liquid crystal layer is zero, resultsof display conditions when white tone is being displayed by setting avalue that derives 100% of the transmittance along the normal for eachsample cell.

[0154] Table 1 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance whenwhite tone is being displayed to be 100%, the sample cells #4 and #5,having respective pretilt angles of 5.0° and 10.0°, displayed highquality images with no tone reversion being observed in the oppositeviewing direction at a viewing angle of 70°.

[0155] Up to a viewing angle of 60°, the sample cells #2 and #3, havingrespective pretilt angles of 3.0° and 4.0°, displayed high qualityimages with no tone reversion being observed in the opposite viewingdirection. At a viewing angle of 70°, tone reversion was observed withthe sample cell #2 within the extent that did not pose any problem forreal use, and tone was distorted, although not reversed, with the samplecell #3. The sample cells #3 and #4 however did not pose any problem forreal use at the viewing angle of 70°.

[0156] Up to a viewing angle of 60°, the sample cell #6 with the pretiltangle of 11.0° displayed high quality images. However, at a viewingangle of 70°, a decrease in luminance was evident in the standardviewing direction to the extent unbearable for real use.

[0157] With the sample cell #1, having a pretilt angle of 2.0°, tonereversion was observed in the opposite viewing direction at a viewingangle as low as 50°. With the sample cell #7, having a pretilt angle of12.0°, a decrease in luminance was evident in the standard viewingdirection at a viewing angle as low as 50° to the extent unbearable forreal use. TABLE 2 Applied voltage for white tone set to derive atransmittance 97% that in the OFF state Viewing Pretilt angle (°) Angle2.0 3.0 4.0 5.0 10.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘x₂ 60° x₁ Δ₁ ∘ ∘ ∘ x₂ x₂ 70° x₁ x₁ ∘ ∘ ∘ x₂ x₂ # in the standard viewingdirection to the extent unbearable for real use.)

[0158] Table 2 shows results observed by setting a voltage for whitetone for each sample cell to cause the transmittance for white tone tobe 97% that in an OFF state.

[0159] Table 2 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance whenwhite tone was being displayed to be 97%, the sample cells #3, #4, and#5, having respective pretilt angles of 4.0°, 5.0°, and 10.0°, displayedhigh quality images with no tone reversion being observed in theopposite viewing direction at a viewing angle of 70°.

[0160] Up to a viewing angle of 50°, the sample cell #2, having apretilt angle of 3.0°, displayed high quality images with no tonereversion being observed in the opposite viewing direction. At a viewingangle of 60°, tone was distorted with the sample cell #2. However, thesample cell #2 did not pose any problem for real use, because tone wasnot reversed. The sample cell #6 with the pretilt angle of 11.0°displayed high quality images up to a viewing angle of 50°. However, ata viewing angle of 60°, a decrease in luminance was evident in thestandard viewing direction to the extent unbearable for real use.

[0161] With the sample cell #1, having a pretilt angle of 2.0°, tonereversion was observed in the opposite viewing direction at a viewingangle as low as 50°. With the sample cell #7, having a pretilt angle of12.0°, a decrease in luminance was evident in the standard viewingdirection at a viewing angle as low as 50° to the extent unbearable forreal use. TABLE 3 Applied voltage for white tone set to derive atransmittance 95% that in the OFF state Viewing Pretilt angle (°) Angle2.0 3.0 4.0 5.0 10.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘x₂ 60° x₁ Δ₁ ∘ ∘ ∘ x₂ x₂ 70° x₁ x₁ ∘ ∘ ∘ x₂ x₂ # in the standard viewingdirection to the extent unbearable for real use.)

[0162] Table 3 shows results observed by setting a voltage for whitetone for each sample cell to cause the transmittance to be 95% that inan OFF state. Those results were the same as in Table 2 in which thevoltage was set to cause the transmittance for white tone to be 97%.TABLE 4 Applied voltage for white tone set to derive a transmittance 92%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₂ ∘ ∘ ∘ ∘ ∘ x₂ 60° x₁ ∘ ∘ ∘∘ x₂ x₂ 70° x₁ Δ₂ ∘ ∘ ∘ x₂ x₂ # in the standard viewing direction to theextent unbearable for real use.)

[0163] Table 4 shows results observed by setting a voltage for whitetone for each sample cell to cause the ratio of the transmittance forwhite tone to be 92% that in an OFF state.

[0164] Table 4 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance forwhite tone to be 92%, the sample cells #3, #4, and #5, having respectivepretilt angles of 4.0°, 5.0°, and 10.0°, displayed high quality imageswith no tone reversion being observed in the opposite viewing directionat a viewing angle of 70°.

[0165] Up to a viewing angle of 60°, the sample cell #2, having apretilt angle of 3.0°, displayed high quality images with no tonereversion being observed in the opposite viewing direction. At a viewingangle of 70°, tone was reversed with the sample cell #2. However, thetone reversion was within the extent that did not pose any problem forreal use. The sample cell #6 with the pretilt angle of 11.0° displayedhigh quality images up to a viewing angle of 50°. However, at a viewingangle of 60°, a decrease in luminance was evident in the standardviewing direction to the extent unbearable for real use. Tone reversionwas observed at a viewing angle of 50° with the sample cell #1, having apretilt angle of 2.0°, within the extent that did not pose any problemfor real use.

[0166] With the sample cell #7, having a pretilt angle of 12.0°, adecrease in luminance was evident in the standard viewing direction at aviewing angle as low as 50° to the extent unbearable for real use. TABLE5 Applied voltage for white tone set to derive a transmittance 90% thatin the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.0 10.011.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₁ ∘ ∘ ∘ ∘ Δ₃ x₂ 60° Δ₂ ∘ ∘ ∘ Δ₃x₂ x₂ 70° x₁ Δ₁ ∘ ∘ x₂ x₂ x₂ # opposite viewing direction within theextent that did not pose any problem for real use, “Δ₃” represents thata decrease in luminance was observed in the standard viewing directionwithin the extent that did not pose any problem for real use, “x₁”represents that tone reversion was observed in the opposite viewingdirection, and “x₂” represents that a decrease in luminance was evidentin # the standard viewing direction to the extent unbearable for realuse.)

[0167] Table 5 shows results observed by setting a voltage for whitetone for each sample cell to cause the ratio of the transmittance forwhite tone to be 90% that in an OFF state.

[0168] Table 5 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance whenwhite tone was being displayed to be 90%, the sample cells #3 and #4,having respective pretilt angles of 4.0° and 5.0°, displayed highquality images with no tone reversion being observed in the oppositeviewing direction at a viewing angle of 70°.

[0169] Up to a viewing angle of 50°, the sample cell #5, having apretilt angle of 10.0°, displayed high quality images. At a viewingangle of 60°, a decrease in luminance was observed in the standardviewing direction within the extent that did not pose any problem forreal use. Up to a viewing angle of 60°, the sample cell #2, having apretilt angle of 3.0°, displayed high quality images with no tonereversion being observed even in the opposite viewing direction. At aviewing angle of 70°, tone was distorted within the extent that did notpose any problem for real use, but no tone reversion was observed. Withthe sample cell #6 with the pretilt angle of 11.0°, a decrease inluminance was observed in the standard viewing direction at a viewingangle of 50° within the extent that did not pose any problem for realuse. With the sample cell #1, having a pretilt angle of 2.0°, tone wasdistorted at a viewing angle of 50° and reversed at a viewing angle of60° within the extent that did not pose any problem for real use.

[0170] With the sample cell #7, having a pretilt angle of 12°, adecrease in luminance was evident in the standard viewing direction at aviewing angle as low as 50° to the extent unbearable for real use. TABLE6 Applied voltage for white tone set to derive a transmittance 87% thatin the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.0 10.011.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° ∘ ∘ ∘ ∘ Δ₃ x₂ x₂ 60° Δ₃ Δ₃ Δ₃ Δ₃Δ₃ x₂ x₂ 70° x₂ x₂ x₂ x₂ x₂ x₂ x₂

[0171] Table 6 shows results observed by setting a voltage for whitetone for each sample cell to cause the ratio of the transmittance forwhite tone to be 87% that in an OFF state.

[0172] Table 6 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance whenwhite tone was being displayed to be 87%, the sample cells #2, #3, and#4, having respective pretilt angles of 3.0°, 4.0° and 5.0°, displayedhigh quality images up to a viewing angle of 50°. However, at a viewingangle of 60°, a decrease in luminance was observed in the standardviewing direction within the extent that did not pose any problem forreal use. At a viewing angle of 70°, a decrease in luminance was evidentin the standard viewing direction to the extent unbearable for real use.

[0173] At viewing angles of 50° and 60°, a decrease in luminance wasobserved with the sample cell #5, having a pretilt angle of 10.0°, inthe standard viewing direction within the extent that did not pose anyproblem for real use. At a viewing angle of 70°, a decrease in luminancewas evident in the standard viewing direction to the extent unbearablefor real use.

[0174] With the sample cells #6 and #7, having respective pretilt anglesof 11.0° and 12.0°, a decrease in luminance was evident in the standardviewing direction at a viewing angle as low as 50° to the extentunbearable for real use.

[0175] Up to a viewing angle of 50°, the sample cell #1, having apretilt angle of 2.0°, displayed high quality images with no tonereversion being observed in the opposite viewing direction. However, ata viewing angle of 60°, a decrease in luminance was observed in thestandard viewing direction within the extent that did not pose anyproblem for real use. At a viewing angle of 70°, a decrease in luminancewas evident in the standard viewing direction to the extent unbearablefor real use. TABLE 7 Applied voltage for white tone set to derive atransmittance 85% that in the OFF state Viewing Pretilt angle (°) Angle2.0 3.0 4.0 5.0 10.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₃ x₃ x₃ x₃x₃ x₃ x₃ 60° x₃ x₃ x₃ x₃ x₃ x₃ x₃ 70° x₃ x₃ x₃ x₃ x₃ x₃ x₃

[0176] Table 7 shows results observed by setting a voltage for whitetone for each sample cell to cause the ratio of the transmittance forwhite tone to be 85% that in an OFF state.

[0177] Table 7 shows that in a case where the voltage when white tonewas being displayed was set to cause the ratio of the transmittance whenwhite tone was being displayed to be 85%, a decrease in luminance wasevident with the sample cells #2, #3, #4, #5, #6, and #7, havingrespective pretilt angles of 3.0°, 4.0° 5.0°, 10.0°, 11.0° and 12.0°, inthe standard viewing direction and in the right- and left-handdirections at a viewing angle as low as 50° to the extent unbearable forreal use.

[0178] At a viewing angle of 50°, a decrease in luminance was observedwith the sample cell #1, having a pretilt angle of 2.0°, in the standardviewing direction within the extent that did not pose any problem forreal use. At a viewing angle of 60°, a decrease in luminance was evidentin the standard viewing direction to the extent unbearable for real use.

[0179] It can be concluded from Tables 1 to 7 that tone reversion can berestrained in the opposite viewing direction by adjusting the pretiltangle or the transmittance when white tone is being displayed. It can bealso concluded that in such an event, at a value ranging from 95% to 97%to which the ratio of the transmittance is normally set as thetransmittance for white tone, the setting of the pretilt angle in arange larger than 2° and smaller than 12° permits high quality images tobe displayed at a viewing angle of 50°0 with tone reversion beingrestrained in the opposite viewing direction and no decrease inluminance being observed in the standard viewing direction. It can befurther concluded that the setting of the pretilt angle in a range notless than 4° and not more than 10° permits high quality images to bedisplayed at a wide viewing angle of 70° with tone reversion beingrestrained in the opposite viewing direction and no decrease inluminance being observed in the standard viewing direction.

[0180] Moreover, it can be concluded that at a pretilt angle of 2° to10°, to which the pretilt angle is normally set, such setting that atransmittance not higher than 85% is derived as the transmittance whenwhite tone is being displayed permits high quality images to bedisplayed at a viewing angle of 50° with tone reversion being restrainedin the opposite viewing direction and no decrease in luminance beingobserved in the standard viewing direction. It can be also concludedthat such setting that a transmittance within a range not less than 90%and not more than 97% is derived, plus the adjustment of the pretiltangle, permits high quality images to be displayed at a wide viewingangle of 70° with tone reversion being restrained in the oppositeviewing direction and no decrease in luminance being observed in thestandard viewing direction.

[0181] Moreover, it can be concluded that a combination of theadjustment of the pretilt angle and that of the transmittance when whitetone is being displayed further enhances the effects of improvement.

[0182] Next, viewing angle dependency of the liquid crystal displaydevice was checked with the same samples #1 and #4 as above by using ameasuring system including a light receiving element 21, an amplifier22, and a recording device 23 as shown in FIG. 6.

[0183] In this measuring system, the liquid crystal cell 16 of theliquid crystal display device is placed so that the surface 16 a facingthe glass substrate 9 lies on the reference plane X-Y of the rectangularcoordinates XYZ. The light receiving element 21 is an element capable ofreceiving light at a certain solid light receiving angle, and is locateda predetermined distance away from the original point of the coordinatesat an angle (viewing angle) of Φ with respect to the Z-directionorthogonal to the plane 16 a.

[0184] Upon measurement, monochromatic light having a wavelength of 550nm is emitted from the surface opposite the plane 16 a to irradiate theliquid crystal cell 16 in the measuring system. Part of themonochromatic light having passed through the liquid crystal cell 16enters the light receiving element 21. Output by the light receivingelement 21 is amplified to a predetermined level by the amplifier 22,and recorded in the recording device 23, such as a waveform memory or arecorder.

[0185] Here, the output level by the light receiving element 21 inresponse to the applying of voltage to the sample cells #1 and #4 wasmeasured with the light receiving element 21 being fixed at a certainangle Φ.

[0186] The measurement was done, assuming that the Y-direction is theleft-hand side of the screen and the X-direction is the downwarddirection (standard viewing direction) of the screen, while disposingthe light receiving element 21 in the upward direction (opposite viewingdirection), the downward direction (standard viewing direction), and theright- and left-hand directions with the angle θ being maintained at50°.

[0187] Graphs in FIGS. 7(a) to 7(c) show results, illustrating thebehavior of the light transmittances of the sample cells #4 and #1,having respective pretilt angles of 5.0° and 2.0°, in response tovoltage applied thereto, that is, the transmittance versus liquidcrystal applied voltage characteristics.

[0188]FIG. 7(a) shows results of the measurement from the upwarddirection in FIG. 2. FIG. 7(b) shows results of the measurement from thedownward direction in FIG. 2. FIG. 7(c) shows results of the measurementfrom the right- and left-hand directions in FIG. 2.

[0189] Referring to FIG. 7(a), the curved alternative long and shortdash line L1 represents results of measurement in the front direction,i.e. the direction normal to the surface. Both the sample cell #1 andthe sample cell #4 exhibit the same transmittance versus liquid crystalapplied voltage characteristics.

[0190] Referring to FIGS. 7(a) to 7(c), the solid lines L2, L4, and L6represent the sample cell #4, and the dotted lines L3, L5, and L7represent the sample #1.

[0191] To compare the sample cell #4 with the sample cell #1 in terms oftransmittance versus liquid crystal applied voltage characteristics inthe upward direction in FIG. 7(a), the curved line L3 for the samplecell #1 has a bumpy shape, or rise and fall of the transmittance,between about 1 V and 2 V. By contrast, the curved line L2 for thesample cell #4 is flat between about 1 V and 2 V with the transmittancestaying at a value, and has no bumpy shape, showing that the sample cell#4 is free from the tone reversion phenomenon.

[0192] To compare those sample cells in terms of transmittance versusliquid crystal applied voltage characteristics in the downward,left-hand, and right-hand directions in FIGS. 7(b) and 7(c), the curvedlines L4 and L6 for the sample cell #4 and the curved lines L5 and L7for the sample cell #1 show that the transmittance of the sample cell #4drops a little more quickly than that of the sample cell #1. However,the transmittance of the sample cell #4 starts to conform to that of thesample cell #1 at around 2.5 V in FIG. 7(b) and at around 3 V in FIG.7(c). Therefore, it can be confirmed that the larger pretilt angleequalling 5.0° has no adverse effects.

[0193] The same results were obtained with sample cells prepared in thesame manner as the sample cells #1 to #7 except that those sample cellseach include optical retardation compensator plates 2 and 3 composed ofdiscotic liquid crystal treated with hybrid orientation on a transparentsupport base.

SECOND EXAMPLE

[0194] The present example is to substantiate the effects of the liquidcrystal display devices in accordance with the first to thirdembodiments. Here, three sample cells #16 to #18 were prepared by usingOptomer AL (product name), available from Japan Synthetic Rubber Co.,Ltd., as the orientation films 11 and 14 of the liquid crystal cell 16of the liquid crystal display device shown in FIG. 1, using as theliquid crystal layer 8 liquid crystal materials of which the pretiltangle is 3° and of which the refractive index anisotropies Δn_(L)(550)at a wavelength of 550 nm are 0.070, 0.080, and 0.095 respectively, andsetting the thickness of the cells (of the liquid crystal layers 8) to 5μm.

[0195] In the same manner as in the previous example, homogeneous cellswere prepared by injecting thereinto the materials for the sample cells#16 to #18, and measured with a pretilt angle measuring device,NSMAP-3000LCD, for the pretilt angles of the sample cells #16 to #18.

[0196] Used as the optical retardation compensator plates 2 and 3 of thesample cells #16 to #18 are the optical retardation compensator plates 2and 3 of the same kind as those in the first example above includingdiscotic liquid crystal treated with an oblique orientation technique.

[0197] The same measuring system as that in the first example aboveshown in FIG. 6 was used to measure the output level by the lightreceiving element 21 in response to the applying of voltage to thesample cells #16 to #18 with the light receiving element 21 being fixedat a certain angle θ.

[0198] The measurement was done, assuming that the Y-direction is theleft-hand side of the screen and the X-direction is the downwarddirection (standard viewing direction) of the screen, while disposingthe light receiving element 21 in the upward direction (opposite viewingdirection), the downward direction (standard viewing direction), and theright- and left-hand directions with the angle θ being maintained at50°.

[0199] Graphs in FIGS. 8(a) to 8(c) show results, illustrating thebehavior of light transmittance of the sample cells #16 to #18 inresponse to voltage applied thereto, that is, the transmittance versusliquid crystal applied voltage characteristics.

[0200]FIG. 8(a) shows results of the measurement from the upwarddirection in FIG. 2. FIG. 8(b) shows results of the measurement from theright-hand direction in FIG. 2. FIG. 8(c) shows results of themeasurement from the left-hand direction in FIG. 2.

[0201] Referring to FIGS. 8(a) to 8(c), the curved alternative long andshort dash lines L8, L11, and L4 represent the sample cell #16 using aliquid crystal material of Δn_(L)(550)=0.070 for the liquid crystallayer 8, the solid lines L9, L12, and L15 represent the sample cell #17using a liquid crystal material of Δn_(L)(550)=0.080 for the liquidcrystal layer 8, and the dotted lines L10, L13, and L16 represent thesample cell #18 using a liquid crystal material of Δn_(L)(550)=0.095 forthe liquid crystal layer 8.

[0202] Two comparative sample cells #103 and #104 were also prepared asa comparative example for the present example in the same manner as thesample cells of the present example except that those comparative samplecells use liquid crystal materials of which the refractive indexanisotropies Δn_(L)(550) at a wavelength of 550 nm are 0.060 and 0.120as the liquid crystal layer 8 of the liquid crystal cell 16 shown inFIG. 1. The measuring system shown in FIG. 6 was used to measure theoutput level by the light receiving element 21 in response to theapplying of voltage to the comparative sample cells #103 and #104 withthe light receiving element 21 being fixed at a certain angle θ in thesame manner as in the present example.

[0203] The measurement was done, assuming that the Y-direction is theleft-hand side of the screen and the X-direction is the downwarddirection (standard viewing direction) of the screen, while disposingthe light receiving element 21 in the upward direction (opposite viewingdirection) and the right- and left-hand directions with the angle θbeing maintained at 50°.

[0204] Graphs in FIGS. 9(a) to 9(c) show results, illustrating thebehavior of light transmittance of the comparative sample cells #103 to#104 in response to voltage applied thereto, that is, the transmittanceversus liquid crystal applied voltage characteristics.

[0205]FIG. 9(a) shows results of the measurement from the upwarddirection in FIG. 2. FIG. 9(b) shows results of the measurement from theright-hand direction in FIG. 2. FIG. 9(c) shows results of themeasurement from the left-hand direction in FIG. 2.

[0206] Referring to FIGS. 9(a) to 9(c), the solid curved lines L17, L19,and L21 represent the comparative sample cell #103 using a liquidcrystal material having Δn_(L)(550) of 0.060 for the liquid crystallayer 8, and the dotted curved lines L18, L20, and L22 represent thecomparative sample cell #104 using a liquid crystal material havingΔn_(L)(550) of 0.120 for the liquid crystal layer 8.

[0207] To compare the sample cells #16 to #18 and the comparative samplecells #103 and #104 in terms of transmittance versus liquid crystalapplied voltage characteristics in the upward direction in FIGS. 8(a)and 9(a), the curved lines L9, L8, and L10 show that the transmittancesdrop by sufficient amounts with higher voltages. By contrast, incomparison with the curved lines L8, L9, and L10, the curved line L18shows that the transmittance does not drop sufficiently with highervoltages, and the curved line L17 shows that the transmittance drops andthen rises with higher voltages, resulting in tone reversion phenomenon.

[0208] To compare the sample cells #16 to #18 and the comparative samplecells #103 and #104 in terms of transmittance versus liquid crystalapplied voltage characteristics in the right-hand direction in FIGS.8(b) and 9(b), the curved lines L11, L12, and L13 show that thetransmittances drop almost to zero with higher voltages. The curved lineL19 shows that the transmittance drops almost to zero with highervoltages as in FIG. 8(b), while the curved line L20 shows that tonereversion phenomenon occurs. The same results as in the right-handdirection were obtained in the left-hand direction with the sample cells#16 to #18 and the comparative sample cells #103 and #104.

[0209] Visual observations were conducted of the sample cells #16 to #18and the comparative sample cells #103 and #104 under white light.

[0210] The sample cells #16 to #18 and the comparative sample cell #103showed coloration in no direction at a viewing angle of 50°, displayinggood images. By contrast, the comparative sample cell #104 showedcoloration ranging from yellow to orange in the right- and left-handdirections at a viewing angle of 50°.

[0211] It can be concluded from those results shown in FIGS. 8(a) to8(c) that if the liquid crystal layer 8 is made of a liquid crystalmaterial of which the refractive index anisotropy Δn_(L)(550) at awavelength of 550 nm is 0.070, 0.080, or 0.095, the transmittance dropsby a sufficient amount with higher voltages, thereby shows no tonereversion phenomenon, expanding the effective viewing angle, and showsno coloration phenomenon, greatly improving the display quality of theliquid crystal display device.

[0212] It can be concluded, on the other hand, from those results inFIGS. 9(a) to 9(c) that if the liquid crystal layer 8 is made of aliquid crystal material of which the refractive index anisotropyΔn_(L)(550) at a wavelength of 550 nm is 0.060 or 0.120, the viewingangle dependency is not restrained satisfactorily.

[0213] The same results were obtained with sample cells and comparativesample cells prepared in the same manner as the sample cells #16 to #18and the comparative sample cells #103 and #104 except that those samplecells and comparative sample cells include optical retardationcompensator plates 2 and 3 composed of discotic liquid crystal treatedwith hybrid orientation on a transparent support base.

[0214] The transmittance versus liquid crystal applied voltagecharacteristics were examined for the dependency thereof upon theinclination angle θ of the refractive index ellipsoid of the opticalretardation compensator plates 2 and 3, by varying the inclination angleθ. The results were such that the transmittance versus liquid crystalapplied voltage characteristics remained virtually unchanged irrelevantto the orientation state of the discotic liquid crystal of the opticalretardation compensator plates 2 and 3, as long as the inclination angleθ stayed in the range of 15°≦θ≦75°. It was also observed that when theinclination angle θ was varied out of that range, the effective viewingangle did not become wider in the opposite viewing direction.

[0215] The transmittance versus liquid crystal applied voltagecharacteristics were examined for the dependency thereof upon the secondretardation value of the optical retardation compensator plates 2 and 3,by varying the second retardation value. The results were such that thetransmittance versus liquid crystal applied voltage characteristicsremained virtually unchanged irrelevant to the orientation state of thediscotic liquid crystal of the optical retardation compensator plates 2and 3, as long as the second retardation value stayed in the range of 80nm to 250 nm. It was also observed that when the second retardationvalue was varied out of that range, the effective viewing angle did notbecome wider in the opposite viewing direction.

[0216] In light of the results of the visual observations of thecomparative sample cells #103 and #104, three sample cells #19 to #21were prepared in the same manner as in the present example except thatthe sample cells #19 to #21 used liquid crystal materials of which therefractive index anisotropies Δn_(L)(550) at a wavelength of 550 nm are0.065, 0.100, and 0.115 as the liquid crystal layer 8 of the liquidcrystal cell 16 shown in FIG. 1. The measuring system shown in FIG. 6was used to measure the output level by the light receiving element 21in response to the applying of voltage to the sample cells #19 to #21with the light receiving element 21 being fixed at a certain angle θ inthe same manner as in the present example. Visual observations were alsoconducted of the sample cells #19 to #21 under white light.

[0217] The results show that the transmittance of the sample cell #20with the refractive index anisotropy Δn_(L)(550) of 0.100 and that ofthe sample cell #21 with the refractive index anisotropy Δn_(L)(550) of0.115 rose slightly with higher voltages in the right- and left-handdirections with the angle θ of 50°. However, no tone reversionphenomenon was visually confirmed, and those rises in the transmittanceswere within the extent that did not pose any problem for real use. Theresults show no problem at all in the upward direction. Meanwhile,similarly to the transmittance of the aforementioned comparative samplecell #103, the transmittance of the sample cell #19 with the refractiveindex anisotropy Δn_(L)(550) of 0.065 dropped slightly and then rosewith higher voltages in the upward direction. However, the rise in thetransmittance was relatively small as compared with that of thecomparative sample cell #103 shown in FIG. 9(a), being within the extentthat did not pose any problem for real use. The results show no problemat all in the right- and left-hand directions.

[0218] Visual observation discovered slight coloration ranging fromyellow to orange with the sample cells #20 and #21, however, within theextent that did not pose any problem for real use. Visual observationalso discovered slight bluish coloration with the sample cell #19,however, within the extent that did not pose any problem for real use.

[0219] As a supplement, the sample cell #19 and the comparative samplecell #103 were measured for transmittances when white tone was beingdisplayed in the direction normal to the surface of the liquid crystalcell 16, by applying a voltage of about 1 V. The results show that thetransmittance of the comparative sample cell #103 dropped to the extentunbearable for real use, while the transmittance of the sample cell #19dropped slightly, however, within the extent that did not pose anyproblem for real use.

[0220] The same results were obtained in a case where Optomer AL(product name), available from Japan Synthetic Rubber Co., Ltd., wasused as the orientation films 11 and 14 of the liquid crystal cell 16 ofthe liquid crystal display device shown in FIG. 1, and liquid crystalmaterials that formed pretilt angles of 4°, 5°, 10°, and 11° to theorientation films 11 and 14 were used as the liquid crystal layer 8.

THIRD EXAMPLE

[0221] The present example is to substantiate the effects of the liquidcrystal display device in accordance with the third embodiment. Here,five sample cells #31 to #35 were prepared by using, as the liquidcrystal layer 8 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1, liquid crystal materials and opticalretardation compensator plates of which the relations described in theexpression (1) were set to 0, 0.15, 0.25, 0.30, and 0.33 respectively,and setting the thickness of the cells (of the liquid crystal layers 8)to 5 μm, the relation concerning the refractive index anisotropyΔn_(L)(450) of the liquid crystal layer 8 at a wavelength of 450 nm, therefractive index anisotropy Δn_(L)(550) thereof at a wavelength of 550nm, the refractive index anisotropy Δn_(F)(450) of the opticalretardation compensator plates 2 and 3 at a wavelength of 450 nm, therefractive index anisotropy Δn_(F)(550) thereof at a wavelength of 550nm. $\begin{matrix}\frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} & (1)\end{matrix}$

[0222] Used as the optical retardation compensator plates 2 and 3 of thesample cells #31 to #35 are those constituted by a transparent supportbase (e.g., triacetylcellulose (TAC)) on which discotic liquid crystalis applied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

[0223] As a comparative example for the present example, comparativesample cells #301 to #303 were also prepared in the same manner as thepresent example except that liquid crystal materials and opticalretardation compensator plates of which the relations described in theexpression (1) equaled 0.35, 1.0, and 1.1 were used as the liquidcrystal layer 8 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1.

[0224] Table 8 shows results of visual observations of the sample cells#31 to #35 and the comparative sample cells #301 to #303 under whitelight. TABLE 8$\frac{{{{\Delta n}_{L}(450)}/{{\Delta n}_{L}(550)}} - 1}{{{{\Delta n}_{F}(450)}/{{\Delta n}_{F}(550)}} - 1}$

Viewing 0 0.15 0.25 0.30 0.33 0.35 1.0 1.1 Angle (θ) #31 #32 #33 #34 #35#301 #302 #303 50° ∘ ∘ ∘ ∘ ∘ × × × 60° ∘ ∘ ∘ Δ × × × × 70° ∘ ∘ ∘ × × × ××

[0225] The sample cells #31 to #33 displayed good images with colorationbeing observed in no direction at a viewing angle of 70°. The samplecell #34 displayed good images with coloration being observed in nodirection at viewing angles up to 50°, however, displaying slightcoloration within the extent that did not pose any problem for real usein the right- and left-hand directions at a viewing angle of 60°. Thesample cell #35 displayed good images with coloration being observed inno direction at viewing angles up to 50°, however, displaying colorationto the extent unbearable for real use in the right- and left-handdirections at a viewing angle of 60°.

[0226] By contrast, the comparative sample cells #301 to #303 displayedcoloration ranging from yellow to orange to the extent unbearable forreal use in the right- and left-hand directions at a viewing angle aslow as 50°.

[0227] The same results were obtained with sample cells and comparativesample cells prepared in the same manner as the sample cells #31 to #35and the comparative sample cells #301 and #303 except that those samplecells and comparative sample cells included optical retardationcompensator plates 2 and 3 composed of discotic liquid crystal treatedwith hybrid orientation on a transparent support base.

FOURTH EXAMPLE

[0228] The present example is to substantiate the effects of the liquidcrystal display device in accordance with the third embodiment. Here,five sample cells #41 to #45 were prepared by using, as the liquidcrystal layer 8 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1, liquid crystal materials and opticalretardation compensator plates of which the relations described in theexpression (2) were set to 0, 0.10, 0.20, 0.23, and 0.25 respectively,and setting the thickness of the cells (of the liquid crystal layers 8)to 5 μm, the relation concerning the refractive index anisotropyΔn_(L)(550) of the liquid crystal layer 8 at a wavelength of 550 nm, therefractive index anisotropy Δn_(L)(650) thereof at a wavelength of 650nm, the refractive index anisotropy Δn_(F)(550) of the opticalretardation compensator plates 2 and 3 at a wavelength of 550 nm, therefractive index anisotropy Δn_(F)(650) thereof at a wavelength of 650nm. $\begin{matrix}\frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} & (2)\end{matrix}$

[0229] Used as the optical retardation compensator plates 2 and 3 of thesample cells #41 to #45 are those constituted by a transparent supportbase (e.g., triacetylcellulose (TAC)) on which discotic liquid crystalis applied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

[0230] As a comparative example for the present example, comparativesample cells #401 to #403 were also prepared in the same manner as thepresent example except that liquid crystal materials and opticalretardation compensator plates of which the relations described in theexpression (2) equaled 0.27, 1.0, and 1.1 were used as the liquidcrystal layer 8 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1.

[0231] Table 9 shows results of visual observations of the sample cells#41 to #45 and the comparative sample cells #401 to #403 under whitelight. TABLE 9$\frac{1 - {{{\Delta n}_{L}(450)}/{{\Delta n}_{L}(550)}}}{1 - {{{\Delta n}_{F}(450)}/{{\Delta n}_{F}(550)}}}$

Viewing 0 0.10 0.20 0.23 0.25 0.27 1.0 1.1 Angle (θ) #41 #42 #43 #44 #45#401 #402 #403 50° ∘ ∘ ∘ ∘ ∘ × × × 60° ∘ ∘ ∘ Δ × × × × 70° ∘ ∘ ∘ × × × ××

[0232] The sample cells #41 to #43 displayed good images with colorationbeing observed in no direction at a viewing angle of 70°. The samplecell #44 displayed good images with coloration being observed in nodirection at viewing angles up to 50°, however displaying slightcoloration within the extent that did not pose any problem for real usein the right- and left-hand directions at a viewing angle of 60°. Thesample cell #45 displayed slight coloration within the extent that didnot pose any problem for real use in the right- and left-hand directionsat a viewing angle of 50°.

[0233] By contrast, the comparative sample cells #401 to #403 displayedcoloration ranging from yellow to orange to the extent unbearable forreal use in the right- and left-hand directions at a viewing angle aslow as 50°.

[0234] The same results were obtained with sample cells and comparativesample cells prepared in the same manner as the sample cells #41 to #45and the comparative sample cells #401 and #403 except that those samplecells and comparative sample cells included optical retardationcompensator plates 2 and 3 composed of discotic liquid crystal treatedwith hybrid orientation on a transparent support base.

[0235] As explained above, the first arrangement of the presentinvention incorporates, between the liquid crystal layer and thepolarizer, an optical retardation compensator plate represented by arefractive index ellipsoid of which the three principal refractiveindices, na, nb, and nc, are mutually related by the inequalityna=nc>nb, and of which the shorter axis coincident with the principalrefractive index nb inclines with respect to the normal direction of thesurface of the optical retardation compensator plate. Therefore, withthe arrangement, for a case where a linearly polarized ray is convertedto an elliptically polarized ray according to the phase differencebetween the ordinary and extraordinary rays developed from the linearlypolarized ray upon the passing through the liquid crystal layerpossessing birefringence, the optical retardation compensator platecompensates for the phase difference between the ordinary andextraordinary rays that varies depending upon the viewing angle.

[0236] However, the compensation function of this kind still falls shortof satisfying the increasing demand for a better restraint in theviewing angle dependency. Bearing that in mind, the inventors of thepresent invention have conducted further research diligently and foundout that the pretilt angle formed by the orientation films and thelonger axes of liquid crystal molecules in the liquid crystal layeraffects the tone reversion in the opposite viewing direction,especially, when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal, which has led to the completion of the present invention.

[0237] With the liquid crystal display device of the first arrangementin accordance with the present invention, the pretilt angle of theliquid crystal layer sealed in the liquid crystal display element is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal. This can eliminate the tone reversion in the oppositeviewing direction on a screen displaying halftone, and thereby furtherrestrain the viewing angle dependency of the screen. The contrastvariations and coloration are also restrained better than only by thecompensation function by the optical retardation compensator plate.

[0238] The inventors have found that the larger the pretilt angles are,the less likely the tone reversion occurs in the opposite viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal. However, the inventors have also found that too large pretiltangles cause an abrupt decrease in luminance in the standard viewingdirection when halftone is being displayed. Thus, in the firstarrangement above, the abrupt decrease in luminance can be restrained inthe standard viewing direction when halftone is being displayed, byfurther setting the pretilt angle within such a range that luminancedoes not decrease abruptly in the standard viewing direction whenhalftone is being displayed by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal.

[0239] Specifically, the range that does not cause tone reversion in theopposite viewing direction when halftone is being displayed by applyingto the liquid crystal a voltage that is close to the threshold voltagefor the liquid crystal, and that does not cause an abrupt decrease inluminance in the standard viewing direction when halftone is beingdisplayed refers to the setting of the pretilt angle within a rangelarger than 2° and smaller than 12°.

[0240] Although in some instances still incapable of completelyeliminating tone reversion in the opposite viewing direction at theviewing angle of 50°, which is the viewing angle typically required forliquid crystal display devices, the setting enables the liquid crystaldisplay device to be viewed in every direction without problems for realuse.

[0241] The above range, for a case of liquid crystal display deviceswith wider viewing angles such as 70°, refers to the setting of thepretilt angle within a range not smaller than 4° and not larger than10°.

[0242] The setting enables the liquid crystal display device to be freefrom tone reversion in the opposite viewing direction at the viewingangle of 70°, which is the viewing angle typically required for theliquid crystal display device with a wider viewing angle, when halftoneis being displayed.

[0243] For these reasons, with the first arrangement, the contrast ratioin black and white display is not affected by the viewing angle of theobserver, and the quality of images displayed by the liquid crystaldisplay device is greatly improved.

[0244] As explained above, even if a linearly polarized ray is convertedto an elliptically polarized ray according to the phase differencebetween the ordinary and extraordinary rays developed from the linearlypolarized ray upon the passing through the liquid crystal layerpossessing birefringence, the second arrangement, similarly to the firstarrangement, compensates for the phase difference by the opticalretardation compensator plate.

[0245] However, the compensation function of this kind still falls shortof satisfying the increasing demand for a better restraint in theviewing angle dependency. Bearing that in mind, the inventors of thepresent invention have conducted further research diligently and foundout that the value of applied voltage for displaying halftone obtainedby applying to the liquid crystal a voltage that is close to thethreshold voltage for the liquid crystal affects the tone reversion inthe opposite viewing direction when halftone is being displayed, whichhas led to the completion of the present invention.

[0246] With the liquid crystal display device of the second arrangementin accordance with the present invention, the value of applied voltagefor displaying halftone obtained by applying to the liquid crystal avoltage that is close to the threshold voltage for the liquid crystal isset within such a range that tone reversion does not occur in theopposite viewing direction when halftone is being displayed. This caneliminate the tone reversion in the opposite viewing direction with ascreen displaying halftone, and thereby further restrain the viewingangle dependency of the screen. The contrast variations and colorationare also restrained better than only by the compensation function by theoptical retardation compensator plate.

[0247] The voltage for displaying halftone is set in the Normally Whitemode, as an example, by way of the ratio of the transmittance for thewhite tone to the transmittance for the OFF state. The inventors havefound that the lower the transmittance is, the less likely the tonereversion occurs in the opposite viewing direction when white tone isbeing displayed. However, the inventors have also found that too lowtransmittances cause an abrupt decrease in luminance in the standardviewing direction. Thus, in the second arrangement above, the abruptdecrease in luminance can be restrained in the standard viewingdirection when halftone is being displayed, by further setting the valueof applied voltage for displaying halftone obtained by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal within such a range that luminance does not decreaseabruptly in the standard viewing direction when halftone is beingdisplayed.

[0248] Specifically, the range that does not cause tone reversion in theopposite viewing direction when halftone is being displayed by applyingto the liquid crystal a voltage that is close to the threshold voltagefor the liquid crystal, and that does not cause an abrupt decrease inluminance in the standard viewing direction when halftone is beingdisplayed refers to the setting of the value of applied voltage fordisplaying halftone obtained by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal so as toobtain a transmittance higher than 85% that in a bright state (OFFstate) where no voltage is applied to the liquid crystal.

[0249] Although in some instances still incapable of completelyeliminating tone reversion in the opposite viewing direction at theviewing angle of 50°, which is the viewing angle typically required forliquid crystal display devices, the setting enables the liquid crystaldisplay device to be viewed in every direction without problems for realuse.

[0250] The above range, for a case of liquid crystal display deviceswith wider viewing angles such as 70°, refers to the setting of thevalue of applied voltage for displaying halftone obtained by applying tothe liquid crystal a voltage that is close to the threshold voltage forthe liquid crystal so as to obtain a transmittance within a range notless than 90% and not more than 97% that in a bright state (OFF state)where no voltage is applied to the liquid crystal.

[0251] The setting enables the liquid crystal display device to be freefrom tone reversion in the opposite viewing direction at the viewingangle of 70°, which is the viewing angle typically required for theliquid crystal display device with a wider viewing angle, when halftoneis being displayed.

[0252] For these reasons, with the arrangement, the contrast ratio inblack and white display is not affected by the viewing angle of theobserver, and the quality of images displayed by the liquid crystaldisplay device is greatly improved.

[0253] As explained above, even if a linearly polarized ray is convertedto an elliptically polarized ray according to the phase differencebetween the ordinary and extraordinary rays developed from the linearlypolarized ray upon the passing through the liquid crystal layerpossessing birefringence, the arrangement, similarly to the firstarrangement, compensates for the phase difference by the opticalretardation compensator plate.

[0254] However, the compensation function of this kind still falls shortof satisfying the increasing demand for a better restraint in theviewing angle dependency. Bearing that in mind, the inventors of thepresent invention have conducted further research diligently and foundout that the ratios of the variation in the refractive index anisotropy,Δn_(L), of the liquid crystal material for the liquid crystal layer withthe wavelength of light and of the variation in the refractive indexanisotropy, Δn_(F), of the optical retardation compensator plate withthe wavelength of light affect the coloration on the liquid crystalscreen depending upon the viewing angle, which has led to the completionof the present invention.

[0255] With the liquid crystal display device of the third arrangementin accordance with the present invention, the ratios of the variation inthe refractive index anisotropy, Δn_(L), of the liquid crystal materialfor the liquid crystal layer with the wavelength of light and of thevariation in the refractive index anisotropy, Δn_(F), of the opticalretardation compensator plate with the wavelength of light are setwithin such a range that viewing angle dependency does not causecoloration on the liquid crystal screen. This can further restraincoloration on the screen. The contrast variations and tone reversion arealso restrained better than only by the compensation function by theoptical retardation compensator plate.

[0256] The range that does not cause coloration on the liquid crystalscreen depending upon the viewing angle of the above ratio is the rangesatisfying the inequality above.

[0257] Specifically, as described above, the range refers to the settingof the ratio, Δn_(L)(450)/Δn_(L)(550), of the refractive indexanisotropy, Δn_(L)(450), of the liquid crystal material for the liquidcrystal layer to light having a wavelength of 450 nm and the refractiveindex anisotropy, Δn_(L)(550), thereof to light having a wavelength of550 nm, and the ratio, Δn_(F)(450)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(450), of the optical retardation compensator plate tolight having a wavelength of 450 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm so as tosatisfy the inequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} < 0.35$

[0258] Alternatively, as described above, the range refers to thesetting of the ratio, Δn_(L)(650)/Δn_(L)(550), of the refractive indexanisotropy, Δn_(L)(650), of the liquid crystal material for the liquidcrystal layer to light having a wavelength of 650 nm and the refractiveindex anisotropy, Δn_(L)(550), thereof to light having a wavelength of550 nm, and the ratio, Δn_(F)(650)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(650), of the optical retardation compensator plate tolight having a wavelength of 650 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm so as tosatisfy the inequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} < 0.27$

[0259] Although in some instances still incapable of completelyeliminating coloration at the viewing angle of 50°, which is the viewingangle typically required for liquid crystal display devices, the settingof the ratio to fall within either of the above ranges enables theliquid crystal display device to be viewed in every direction withoutproblems for real use.

[0260] The above ranges of the ratio, for a case of, liquid crystaldisplay devices with wider viewing angles such as 70°, are preferablythe ranges satisfying the inequality above.

[0261] That is, as described above, the range refers to the setting ofthe ratio so as to satisfy the inequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} \leq 0.25$

[0262] Alternatively, as described above, the range refers to thesetting of the ratio so as to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} \leq 0.20$

[0263] The setting described above enables the liquid crystal displaydevice to be completely free from coloration phenomenon for everydirection at the viewing angle of 70°, which is the viewing angletypically required for the liquid crystal display device with a widerviewing angle.

[0264] Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with thepresent invention is preferably arranged so that the refractive indexanisotropy, Δn_(L)(550), of the liquid crystal material for the liquidcrystal layer to light having a wavelength of 550 nm is set within arange larger than 0.060 and smaller than 0.120.

[0265] This is because of a confirmation that if the refractive indexanisotropy, Δn_(L)(550), of the liquid crystal material to light havinga wavelength of 550 nm, which is approximately the mid-range of thevisible region of the spectrum, is either not larger than 0.060 or notsmaller than 0.120, tone reversion phenomenon and/or a decrease incontrast ratio occur(s) depending upon the viewing direction. Therefore,the phase difference that occurs to the liquid crystal display elementin accordance with the viewing angle can be eliminated by setting therefractive index anisotropy, Δn_(L)(550), of the liquid crystal materialto light having a wavelength of 550 nm so as to be within a range largerthan 0.060 and smaller than 0.120. This can further restrain thecontrast variations and tone reversion phenomenon in the right- andleft-hand directions, as well as the coloration phenomenon that occursdepending upon the viewing angle.

[0266] In such an event the phase difference that occurs to the liquidcrystal display element in accordance with the viewing angle can be moreeffectively eliminated by setting the refractive index anisotropy,Δn_(L)(550), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 550 nm so as to be within a range notsmaller than 0.070 and not larger than 0.095. This can surely restrainthe contrast variations and tone reversion phenomenon in the right- andleft-hand directions of the images displayed by the liquid crystaldisplay device.

[0267] Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with theinvention is preferably arranged so that the or each optical retardationcompensator plate is represented by a refractive index ellipsoidinclining by an inclination angle set within a range of 15° to 75°.

[0268] By setting the inclination angle of the refractive indexellipsoid to be within a range of 15° to 75° with respect to the or eachoptical retardation compensator plate incorporated in the liquid crystaldisplay device, it is assured that the present invention provides theaforementioned compensation function for the phase difference by theoptical retardation compensator plate.

[0269] Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with theinvention is preferably arranged so that the or each optical retardationcompensator plate has a product, (n_(a)−n_(b))×d, of the differencebetween the principal refractive indices, na and nb, and the thickness,d, of the optical retardation compensator plate, the product being setto be from 80 nm to 250 nm.

[0270] By setting the product, (n_(a)−n_(b))×d, of the differencebetween the principal refractive indices, na and nb, and the thickness,d, of the optical retardation compensator plate, so as to be from 80 nmto 250 nm with respect to the or each optical retardation compensatorplate incorporated in the liquid crystal display device, it is assuredthat the present invention provides the aforementioned compensationfunction for the phase difference by the optical retardation compensatorplate.

[0271] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art intended tobe included within the scope of the following claims.

What is claimed is:
 1. A liquid crystal display device, comprising: aliquid crystal display element formed by sealing a liquid crystal layerbetween a pair of substrates; a pair of polarizers disposed so as toflank the liquid crystal display element; and at least one opticalretardation compensator plate disposed between the liquid crystaldisplay element and the polarizers, the optical retardation compensatorplate being represented by an inclining refractive index ellipsoid,wherein the pretilt angle formed by the orientation films and the longeraxes of liquid crystal molecules in the liquid crystal layer is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal.
 2. The liquid crystal display device as defined in claim1, wherein the refractive index ellipsoid has three principal refractiveindices, na, nb, and nc, mutually related by the inequality na=nc>nb,and inclines as the direction of the principal refractive index nbparallel to the normal to the surface inclines either clockwise orcounterclockwise from the normal direction to the surface around thedirection of the principal refractive index na or nc in the surface. 3.The liquid crystal display device as defined in claim 1, wherein thepretilt angle is further set within such a range that luminance does notdecrease abruptly in the standard viewing direction when halftone isbeing displayed by applying to the liquid crystal a voltage that isclose to the threshold voltage for the liquid crystal.
 4. The liquidcrystal display device as defined in claim 3, the pretilt angle is setwithin a range larger than 2° and smaller than 12°.
 5. The liquidcrystal display device as defined in claim 4, wherein the pretilt angleis set within a range not smaller than 4° and not larger than 10°.
 6. Aliquid crystal display device, comprising: a liquid crystal displayelement formed by sealing a liquid crystal layer between a pair ofsubstrates; a pair of polarizers disposed so as to flank the liquidcrystal display element; and at least one optical retardationcompensator plate disposed between the liquid crystal display elementand the polarizers, the optical retardation compensator plate beingrepresented by an inclining refractive index ellipsoid, wherein a valueof the applied voltage for displaying halftone obtained by applying tothe liquid crystal a voltage that is close to the threshold voltage forthe liquid crystal is set within such a range that tone reversion doesnot occur in the opposite viewing direction when halftone is beingdisplayed.
 7. The liquid crystal display device as defined in claim 6,wherein the refractive index ellipsoid has three principal refractiveindices, na, nb, and nc, mutually related by the inequality na=nc>nb,and inclines as the direction of the principal refractive index nbparallel to the normal to the surface inclines either clockwise orcounterclockwise from the normal direction to the surface around thedirection of the principal refractive index na or nc in the surface. 8.The liquid crystal display device as defined in claim 6, wherein thevalue of the applied voltage is further set within such a range thatluminance does not decrease abruptly in the standard viewing directionwhen halftone is being displayed.
 9. The liquid crystal display deviceas defined in claim 8, wherein the value of the applied voltage is setto obtain a transmittance higher than 85% that in a bright state whereno voltage is applied to the liquid crystal.
 10. The liquid crystaldisplay device as defined in claim 9, wherein the value of the appliedvoltage is set to obtain a transmittance within a range not less than90% and not more than 97% that in a bright state where no voltage isapplied to the liquid crystal.
 11. A liquid crystal display device,comprising: a liquid crystal display element formed by sealing a liquidcrystal layer between a pair of substrates; a pair of polarizersdisposed so as to flank the liquid crystal display element; and at leastone optical retardation compensator plate disposed between the liquidcrystal display element and the polarizers, the optical retardationcompensator plate being represented by an inclining refractive indexellipsoid, wherein the ratios of the variation in the refractive indexanisotropy, Δn_(L), of a liquid crystal material for the liquid crystallayer with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light are set within such arange that viewing angle dependency does not cause coloration on theliquid crystal screen.
 12. The liquid crystal display device as definedin claim 11, wherein the refractive index ellipsoid has three principalrefractive indices, na, nb, and nc, mutually related by the inequalityna=nc>nb, and inclines as the direction of the principal refractiveindex nb parallel to the normal to the surface inclines either clockwiseor counterclockwise from the normal direction to the surface around thedirection of the principal refractive index na or nc in the surface. 13.The liquid crystal display device as defined in claim 11, the ratio,Δn_(L)(450)/Δn_(L)(550), of the refractive index anisotropy,Δn_(L)(450), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 450 nm and the refractive indexanisotropy, Δn_(L)(550), thereof to light having a wavelength of 550 nm,and the ratio, Δn_(F)(450)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(450), of the optical retardation compensator plate tolight having a wavelength of 450 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm satisfy theinequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} < 0.35$


14. The liquid crystal display device as defined in claim 13, whereinΔn_(L)(450)/Δn_(L)(550) and Δn_(F)(450)/Δn_(F)(550) satisfy theinequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} \leq 0.25$


15. The liquid crystal display device as defined in claim 11, whereinthe ratio, Δn_(L)(650)/Δn_(L)(550), of the refractive index anisotropy,Δn_(L)(650), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 650 nm and the refractive indexanisotropy, Δn_(L)(550), thereof to light having a wavelength of 550 nm,and the ratio, Δn_(F)(650)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(650), of the optical retardation compensator plate tolight having a wavelength of 650 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm satisfy theinequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} < 0.27$


16. The liquid crystal display device as defined in claim 15, whereinΔn_(L)(650)/Δn_(L)(550) and Δn_(F)(650)/Δn_(F)(550) satisfy theinequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} \leq 0.20$


17. The liquid crystal display device as defined in claim 1, wherein therefractive index anisotropy, Δn_(L)(550), of a liquid crystal materialfor the liquid crystal layer to light having a wavelength of 550 nm isset within a range larger than 0.060 and smaller than 0.120.
 18. Theliquid crystal display device as defined in claim 6, wherein therefractive index anisotropy, Δn_(L)(550), of a liquid crystal materialfor the liquid crystal layer to light having a wavelength of 550 nm isset within a range larger than 0.060 and smaller than 0.120.
 19. Theliquid crystal display device as defined in claim 11, wherein therefractive index anisotropy, Δn_(L)(550), of the liquid crystal materialfor the liquid crystal layer to light having a wavelength of 550 nm isset within a range larger than 0.060 and smaller than 0.120.
 20. Theliquid crystal display device as defined in claim 17, whereinΔn_(L)(550) is set within a range larger than 0.070 and smaller than0.095.
 21. The liquid crystal display device as defined in claim 18,wherein Δn_(L)(550) is set within a range larger than 0.070 and smallerthan 0.095.
 22. The liquid crystal display device as defined in claim19, wherein Δn_(L)(550) is set within a range larger than 0.070 andsmaller than 0.095.
 23. The liquid crystal display device as defined inclaim 1, wherein the refractive index ellipsoid inclines by aninclination angle set within a range of 15° to 75°.
 24. The liquidcrystal display device as defined in claim 6, wherein the refractiveindex ellipsoid inclines by an inclination angle set within a range of15° to 75°.
 25. The liquid crystal display device as defined in claim11, wherein the refractive index ellipsoid inclines by an inclinationangle set within a range of 15° to 75°.
 26. The liquid crystal displaydevice as defined in claim 1, wherein the or each optical retardationcompensator plate has a product, (n_(a)−n_(b))×d, of the differencebetween the principal refractive indices, na and nb, and the thickness,d, of the optical retardation compensator plate, the product being setto be from 80 nm to 250 nm.
 27. The liquid crystal display device asdefined in claim 6, wherein the or each optical retardation compensatorplate has a product, (n_(a)−n_(b))×d, of the difference between theprincipal refractive indices, na and nb, and the thickness, d, of theoptical retardation compensator plate, the product being set to be from80 nm to 250 nm
 28. The liquid crystal display device as defined inclaim 11, wherein the or each optical retardation compensator plate hasa product, (n_(a)−n_(b))×d, of the difference between the principalrefractive indices, na and nb, and the thickness, d, of the opticalretardation compensator plate, the product being set to be from 80 nm to250 nm.
 29. A liquid crystal display device, comprising: a liquidcrystal display element formed by sealing a liquid crystal layer betweena pair of substrates; a pair of polarizers disposed so as to flank theliquid crystal display element; and at least one optical retardationcompensator plate disposed between the liquid crystal display elementand the polarizers, the optical retardation compensator plate beingrepresented by an inclining refractive index ellipsoid, wherein thepretilt angle formed by the orientation films and the longer axes ofliquid crystal molecules in the liquid crystal layer is set within sucha range that tone reversion does not occur in the opposite viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal, a value of applied voltage for displaying halftone obtained byapplying to the liquid crystal a voltage that is close to the thresholdvoltage for the liquid crystal is set within such a range that tonereversion does not occur in the opposite viewing direction when halftoneis being displayed, and the ratios of the variation in the refractiveindex anisotropy, Δn_(L), of a liquid crystal material for the liquidcrystal layer with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light are set within such arange that viewing angle dependency does not cause coloration on theliquid crystal screen.
 30. A liquid crystal display device, comprising:a liquid crystal display element formed by sealing a90°-twist-orientated liquid crystal layer between a pair of translucentsubstrates, each substrate having a transparent electrode layer and anorientation film on a surface thereof facing the other; a pair ofpolarizers disposed so as to flank the liquid crystal display element;and at least one optical retardation compensator plate disposed betweenthe liquid crystal display element and the polarizers, the opticalretardation compensator plate being represented by a refractive indexellipsoid having three principal refractive indices, na, nb, and nc,mutually related by the inequality na=nc>nb, the refractive indexellipsoid inclining as the direction of the principal refractive indexnb parallel to the normal to the surface and the direction of either theprincipal refractive index na or nc in the surface recline eitherclockwise or counterclockwise around the direction of the principalrefractive index nc or na in the surface, wherein the pretilt angleformed by the orientation films and the longer axes of liquid crystalmolecules in the liquid crystal layer is set within a range larger than2° and smaller than 12°, a value of applied voltage for displayinghalftone obtained by applying to the liquid crystal a voltage that isclose to the threshold voltage for the liquid crystal is set to obtain atransmittance higher than 85% that in a bright state where no voltage isapplied to the liquid crystal, the ratio, Δn_(L)(450)/Δn_(L)(550), ofthe refractive index anisotropy, Δn_(L)(450), of the liquid crystalmaterial for the liquid crystal layer to light having a wavelength of450 nm and the refractive index anisotropy, Δn_(L)(550), thereof tolight having a wavelength of 550 nm, and the ratio,Δn_(F)(450)/Δn_(F)(550), of the refractive index anisotropy,Δn_(F)(450), of the optical retardation compensator plate to lighthaving a wavelength of 450 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm satisfy theinequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} < 0.35$

the ratio, Δn_(L)(650)/Δn_(L)(550), of the refractive index anisotropy,Δn_(L)(650), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 650 nm and the refractive indexanisotropy, Δn_(L)(550), thereof to light having a wavelength of 550 nm,and the ratio, Δn_(F)(650)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(650), of the optical retardation compensator plate tolight having a wavelength of 650 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm satisfy theinequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} < 0.27$


31. The liquid crystal display device as defined in claim 30, whereinthe pretilt angle is set within a range not smaller than 4° and notlarger than 10°.
 32. The liquid crystal display device as defined inclaim 30, wherein the value of the applied voltage is set to obtain atransmittance within a range not less than 90% and not more than 97%that in a bright state where no voltage is applied to the liquidcrystal.
 33. The liquid crystal display device as defined in claim 30,wherein Δn_(L)(450)/Δn_(L)(550) and Δn_(F)(450)/Δn_(F)(550) satisfy theinequality:$0 \leq \frac{\left( {\Delta \quad {{n_{L}(450)}/\Delta}\quad {n_{L}(550)}} \right) - 1}{\left( {\Delta \quad {{n_{F}(450)}/\Delta}\quad {n_{F}(550)}} \right) - 1} \leq 0.25$


34. The liquid crystal display device as defined in claim 30, whereinΔn_(L)(650)/Δn_(L)(550) and Δn_(F)(650)/Δn_(F)(550) satisfy theinequality:$0 \leq \frac{1 - \left( {\Delta \quad {{n_{L}(650)}/\Delta}\quad {n_{L}(550)}} \right)}{1 - \left( {\Delta \quad {{n_{F}(650)}/\Delta}\quad {n_{F}(550)}} \right)} \leq 0.20$